Optical disc apparatus switching focus point between layers

ABSTRACT

A focus jump technique enables focus control on recording layers of a disc in such a manner that its effect is not absorbed by disturbance or a variation in the movement speed of an objective lens. The technique involves monitoring level of a focus error signal and rejecting noise from the error signal. A speed sensor detects movement speed of an objective lens; and a speed control circuit generates a voltage for controlling the objective lens, based on the detected movement speed. Movement speed of the objective lens is detected during focus jump, a corresponding lens drive signal is generated, and an end position is determined from behavior of the error signal immediately before the end of the jump. A focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc apparatus for opticallyreproducing a signal from a disc or optically recording and reproducinga signal on and from a disc. In particular, the invention relates to anoptical disc apparatus capable of reproducing a signal from a dischaving a plurality of recording layers from the disc surface side orrecording and reproducing a signal on and from a disc having a pluralityof recording layers from the disc surface side.

2. Description of the Related Art

Among the currently standardized digital video discs (or digitalversatile discs; hereinafter referred to as “DVDs”) aresingle-surface/single-layer discs, double-surface/single-layer discs,single-surface/double-layer discs, and double-surface/double-layerdiscs. That is, in contrast to other conventional discs such as compactdiscs (hereinafter referred to as “CDs”) and laser discs (hereinafterreferred to as “LDs”) that have only one recording layer on one surface,there are DVDs that have two recording layers on one surface to increasethe recording capacity.

For example, FIG. 2A shows a single-surface/double-layer disc that isproduced by forming a recording layer on each of two 0.6-mm-thick discs,forming a high-reflectance aluminum film and a semitransparent gold filmon the respective discs, and bonding together the two discs. FIG. 2Bshows a double-surface/double-layer disc that is produced by bondingtogether two 0.6-mm-thick discs in each of which information ismultiplexed in the depth direction.

In the above double-layer discs, information is recorded in eachrecording layer. When the level of a signal for driving an objectivelens is increased gradually as shown in FIG. 2D (it is assumed that theobjective lens approaches the disc accordingly), a point (hereinafterreferred to as “focus point”) where the beam is focused at the lowerrecording layer (hereinafter referred to as “0th layer”) occurs in afocus error signal as shown in FIG. 2C at a certain position of theobjective lens. When the objective lens is further elevated, a focuspoint corresponding to the upper recording layer (hereinafter referredto as “first layer”) occurs at a position of the objective lens that ishigher than the previous position. In short, in double-layer discs, thebeam is focused at each recording layer by moving the objective lensvertically. In CDs and LDs, it is sufficient to focus on the singlerecording layer on the single surface. On the other hand, in multi-layerdiscs such as DVDs having two or more planes where information wasrecorded from one side, unless switching is made from a focus pointcorresponding to a recording layer at which the beams is currentlyfocused to another focus point corresponding to another recording layer,information stored in the latter recording layer cannot be read out.

The focus point switching between layers (hereinafter referred to as“focus jump”) is described in Japanese Unexamined Patent PublicationNos. Hei. 9-50630 and Hei. 11-345420, for example. The method disclosedin the publication No. Hei. 9-50630 is as follows. For example, a focusjump from the 0th layer to the first layer is performed as shown in FIG.3. For a move from the 0th layer to the first layer, first, the focusservo loop is rendered in an open state or a hold state and theobjective lens is accelerated by applying an elevation voltage to theactuator for driving the objective lens. In the interlayer region of thefocus position between the 0th layer and the first layer, in a periodwhen the focus error signal is between threshold levels, the voltageapplication to the actuator is stopped. After the focus error signal hasexceeded the upper threshold level, a lowering voltage is applied to theactuator for a prescribed period and the focus servo loop is closed inthe vicinity of the focus point corresponding to the first layer tocomplete the focus jump. This method enables a stable focus jumpirrespective of a variation in interlayer distance, noise that is addedto the focus error signal, a variation in the sensitivity of theactuator for driving the objective lens, and other factors.

The method disclosed in the publication No. Hei. 11-345420 is asfollows. A focus error signal at the end of a deceleration pulse ismeasured in a focus jump, and the output timing of a deceleration pulsein the next focus jump is corrected by using the measured value. Byrepeating this operation, optimum output timing of a deceleration pulsein a focus jump is learned through adjustments. This method enables astable focus jump even in a case where the focus error signal is notbalanced properly or has a distorted waveform due to an offset incircuitry, a local variation in disc characteristics, a variation ininterlayer distance or reflectance, or a variation in pickupcharacteristics and maximum acceleration attained by the actuator issmall.

SUMMARY OF THE INVENTION

However, the above conventional techniques have the following problemsbecause they do not take into account, in performing a focus jump fromone layer at which the beam is focused currently to another layer, aphenomenon called “surface vibration,” a noise component that is addedto the focus error signal, and disturbance that is introduced duringexecution of a focus jump. The surface vibration occurs when the disc isnot completely flat and is warped or curved or the surface of the discmounted is not perpendicular to the rotary shaft of a spindle motorbecause of insufficient mechanical accuracy of a turn table or someother factor.

As shown in FIGS. 4A and 4B, in a state that the servo loop is closed,the voltage of the objective lens drive signal varies according to thesurface vibration component to maintain the focused state. Assume that acertain acceleration voltage is applied to perform a focus jump. Thespeed after the start of acceleration depends on whether theacceleration voltage is added to a valley of the variation of theobjective lens drive signal (see FIG. 4A) or a peak portion thereof (seeFIG. 4B), for the following reasons. In the case of FIG. 4A, theobjective lens moving direction is opposite to the focus jump movementdirection and hence the acceleration caused by the applied accelerationvoltage is small. On the other hand, in the case of FIG. 4B, theobjective lens moving direction is the same as the focus jump movementdirection and hence the acceleration caused by the applied accelerationvoltage is large.

That is, the degree of acceleration caused by the acceleration voltagedepends on the direction in which the objective lens is moving when afocus jump is started. The movement speed of the objective lens whenswitching is made to a focus point corresponding to another recordinglayer and deceleration is started varies and hence the degree ofdeceleration by the deceleration voltage also varies. Since thedeceleration voltage is constant, depending on the speed at the start ofdeceleration, excessive deceleration may cause a return to the focuspoint corresponding to the previous recording layer or insufficientdeceleration may cause passage of the focus point corresponding to thetarget recording layer. This means a problem that it is difficult toperform a focus jump in a stable manner.

The method of the publication No. Hei. 11-345420 solves the aboveproblem in such a manner that an optimum acceleration voltage anddeceleration voltage for a position concerned are learned throughseveral focus jumps to enable a satisfactory focus jump. However, focusjumps are unstable and may fail at a strong possibility until an optimumfocus jump is learned. There is another problem that when the movementspeed of the objective lens is varied by disturbance or the like duringa focus jump, the apparatus cannot absorb influence of the disturbanceand the focus jump becomes unstable because data obtained by learningprovides a constant acceleration voltage and deceleration voltage.

The method of the publication No. Hei. 11-345420 has still anotherproblem that the circuit scale tends to be large because a large amountof learning data needs to be stored to cope with surface vibration and alocal variation in disc characteristics.

A first object of the invention is to provide, by solving the aboveproblems, an optical disc apparatus capable of performing a focus jumpin a stable manner without requiring a large amount of memoryirrespective of influence of surface vibration, a variation ininterlayer distance, noise that is added to the focus error signal, avariation in the sensitivity of the actuator for driving the objectivelens, disturbance that is introduced during a focus jump, and otherfactors.

Previously, DVDs that required a focus jump were ones for reproductiononly (e.g., DVD-ROM (DVD-read only memory) and DVD-VIDEO) on whichlarge-capacity image data of a movie for example, a program, or the likeis recorded in advance. DVDs for recording (e.g., DVD-RAM (DVD-randomaccess memory), DVD-R (DVD-recordable), and DVD-RW (DVD-rewritable)) hadonly a single recording layer and hence did not require a focus jump.

Incidentally, digital broadcasts of high-resolution digital movingpicture data were started recently. To enable long-term recording ofsuch data in usual homes or the like, large-capacity recording media arenecessary. The above-mentioned DVD recording media are insufficient instorage capacity and hence multi-layering of recording discs isindispensable to obtain more capacity.

However, although the capacity is increased by multi-layering arecording surface, a move between layers is needed because of randomaccessibility that is a merit of the medium of disc; it is necessary toperform a focus jump as performed in DVDs for reproduction only. Theabove-described conventional techniques give no consideration to thefocus jump in discs capable of recording data and have the followingproblems that relate to the focus jump to be performed during recording.

Where a recording medium having multiple recording layers is used for,for example, recording a digital broadcast of high-resolution digitalmoving picture data as mentioned above, real-time recording is requiredin which the broadcast is recorded parallel with its reception and henceall-time recording on the recording media should be enabled. Further, inthe case of a disc, there may occur a case that the next recordingposition is distant from the current recording position. Therefore, amove to a target position should be performed instantaneously. Thisresults in a problem that if a move to a target recording layer fails ina focus jump, another attempt should be started from focusing on theoriginal recording layer, which takes time and makes it difficult toenable all-time recording (data is lost during such an attempt). Thereis another problem that a recorded portion may be erased erroneouslyunless a move to a target position is performed correctly duringrecording.

A second object of the invention is to provide, by solving the aboveproblems, an optical disc apparatus capable of performing a focus jumpin a stable manner even during recording in such a manner that inperforming a focus jump not only does the optical disc apparatus controlthe actuator so that the focus position deviates from a target recordinglayer by monitoring whether the level of the focus error signal exceedsa set threshold level, but also it prevents erroneous erasure of data ina recorded portion by decreasing the laser power to such a value thatrecording cannot be effected.

To attain the first object, the invention provides an optical discapparatus having a focus jump function for enabling a focus control oneach of a plurality of recording layers of a disc, comprising anobjective lens for focusing laser light on a recording layer of thedisc; focus error signal generating means for generating a focus errorsignal based on reflection light that is obtained through the objectivelens; generating means for generating, based on the focus error signal,a focus control signal for controlling the objective lens; drive voltagegenerating means for outputting a voltage necessary to move theobjective lens; moving means for moving the objective lens in adirection approximately perpendicular to the recording layers of thedisc in accordance with the output voltage of the drive voltagegenerating means; and speed detecting means for detecting a movementspeed of the objective lens, wherein a movement speed of the objectivelens is detected during a focus jump, a lens drive signal correspondingto the detected movement speed is supplied to the moving means, and anend position of the focus jump is determined based on behavior of thefocus error signal immediately before an end of the focus jump, wherebya focus control is pulled, from a focus point corresponding to onerecording layer, into a focus point corresponding to another recordinglayer forcibly in a stable manner.

Further, the invention provides an optical disc apparatus having a focusjump function for enabling a focus control on each of a plurality ofrecording layers of a disc, comprising an objective lens for focusinglaser light on a recording layer of the disc; focus error signalgenerating means for generating a focus error signal based on reflectionlight that is obtained through the objective lens; generating means forgenerating, based on the focus error signal, a focus control signal forcontrolling the objective lens; drive voltage generating means foroutputting a voltage necessary to move the objective lens; moving meansfor moving the objective lens in a direction approximately perpendicularto the recording layers of the disc in accordance with the outputvoltage of the drive voltage generating means; and means for monitoringa level of the focus error signal, wherein a movement speed of theobjective lens is detected during a focus jump, a lens drive signalcorresponding to the detected movement speed is supplied to the movingmeans, and an end position of the focus jump is determined based onbehavior of the focus error signal immediately before an end of thefocus jump, whereby a focus control is pulled, from a focus pointcorresponding to one recording layer, into a focus point correspondingto another recording layer forcibly in a stable manner.

Further, the invention provides an optical disc apparatus having a focusjump function for enabling a focus control on each of a plurality ofrecording layers of a disc, comprising an objective lens for focusinglaser light on a recording layer of the disc; focus error signalgenerating means for generating a focus error signal based on reflectionlight that is obtained through the objective lens; generating means forgenerating, based on the focus error signal, a focus control signal forcontrolling the objective lens; drive voltage generating means foroutputting a voltage necessary to move the objective lens; moving meansfor moving the objective lens in a direction approximately perpendicularto the recording layers of the disc in accordance with the outputvoltage of the drive voltage generating means; means for monitoring alevel of the focus error signal, speed detecting means for detecting amovement speed of the objective lens; and speed control voltagegenerating means for generating a voltage for controlling the objectivelens based on the movement speed detected by the speed detecting means,wherein a movement speed of the objective lens is detected during afocus jump, a lens drive signal corresponding to the detected movementspeed is supplied to the moving means, and an end position of the focusjump is determined based on behavior of the focus error signalimmediately before an end of the focus jump, whereby a focus control ispulled, from a focus point corresponding to one recording layer, into afocus point corresponding to another recording layer forcibly in astable manner.

Furthermore, the invention provides an optical disc apparatus having afocus jump function for enabling a focus control on each of a pluralityof recording layers of a disc, comprising an objective lens for focusinglaser light on a recording layer of the disc; focus error signalgenerating means for generating a focus error signal based on reflectionlight that is obtained through the objective lens; generating means forgenerating, based on the focus error signal, a focus control signal forcontrolling the objective lens; drive voltage generating means foroutputting a voltage necessary to move the objective lens; moving meansfor moving the objective lens in a direction approximately perpendicularto the recording layers of the disc in accordance with the outputvoltage of the drive voltage generating means; means for rejecting noisefrom the focus error signal; means for monitoring a level of a signalobtained by rejecting the noise form the focus error signal; speeddetecting means for detecting a movement speed of the objective lens;and speed control voltage generating means for generating a voltage forcontrolling the objective lens based on the movement speed detected bythe speed detecting means, wherein a movement speed of the objectivelens is detected during a focus jump, a lens drive signal correspondingto the detected movement speed is supplied to the moving means, and anend position of the focus jump is determined based on behavior of thefocus error signal immediately before an end of the focus jump, wherebya focus control is pulled, from a focus point corresponding to onerecording layer, into a focus point corresponding to another recordinglayer forcibly in a stable manner.

To attain the second object, the invention provides an optical discapparatus having a focus jump function for enabling a focus control oneach of a plurality of recording layers of a disc on and from which datacan be recorded and reproduced, comprising an objective lens forfocusing laser light on a recording layer of the disc; focus errorsignal generating means for generating a focus error signal based onreflection light that is obtained through the objective lens; generatingmeans for generating, based on the focus error signal, a focus controlsignal for controlling the objective lens; drive voltage generatingmeans for outputting a voltage necessary to move the objective lens;moving means for moving the objective lens in a direction approximatelyperpendicular to the recording layers of the disc in accordance with theoutput voltage of the drive voltage generating means; means forcontrolling power of a laser that is used for recording and reproducingdata on and from the disk; means for detecting whether a focus positionof the objective lens will deviate from a recording layer; and controlmeans for starting, when a focus jump becomes necessary during datarecording, the focus jump after switching laser light power that iscurrently made high to enable the data recording to a low power forreproduction, wherein when a focus jump is performed, whether a focusposition of the objective lens will deviate from a destination recordinglayer is detected and deviation from the destination recording layer isprevented by controlling the moving means, whereby the focus jump can beperformed stably during data recording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an optical disc apparatus and a focusjump method according to an embodiment of the present invention;

FIGS. 2A-2D show the structures of two-layer discs and outline a focusjump that is performed in those discs;

FIG. 3 shows an example of an objective lens drive signal for a focusjump in a conventional optical disc apparatus;

FIGS. 4A and 4B show examples of timing, with respect to a surfacevibration component, of applying a voltage for a focus jump in aconventional optical disc apparatus;

FIG. 5 shows a configuration of a pickup shown in FIG. 1 and a specificexample of a signal processing circuit for processing a focus errorsignal;

FIG. 6 is a graph showing how a focus error signal varies depending on adisc deviation;

FIG. 7 is a timing chart showing a specific example of how individualcircuits operate in a focus jump from a 0th-layer focus point to afirst-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 8 is a timing chart showing another specific example of how theindividual circuits operate in a focus jump from a 0th-layer focus pointto a first-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 9 is a timing chart showing a specific example of how theindividual circuits operate in a focus jump from a first-layer focuspoint to a 0th-layer focus point in the optical disc apparatus of FIG.1;

FIG. 10 is a timing chart showing another specific example of how theindividual circuits operate in a focus jump from a first-layer focuspoint to a 0th-layer focus point in the optical disc apparatus of FIG.1;

FIG. 11 shows a specific method for detecting a minimum value of a focuserror signal by means of a value holding circuit shown in FIG. 1;

FIG. 12 shows a specific method for detecting a maximum value of a focuserror signal by means of the value holding circuit shown in FIG. 1;

FIG. 13 is a flowchart showing a specific example of an algorithm offocus jump controls that are performed by a microcomputer in the opticaldisc apparatus of FIG. 1;

FIG. 14 is a block diagram showing an optical disc apparatus and a focusjump method according to another embodiment of the invention;

FIG. 15 is a timing chart showing a specific example of how individualcircuits operate in a focus jump from a 0th-layer focus point to afirst-layer focus point in the optical disc apparatus of FIG. 14;

FIG. 16 is a timing chart showing another specific example of how theindividual circuits operate in a focus jump from a 0th-layer focus pointto a first-layer focus point in the optical disc apparatus of FIG. 14;

FIG. 17 is a timing chart showing a specific example of how theindividual circuits operate in a focus jump from a first-layer focuspoint to a 0th-layer focus point in the optical disc apparatus of FIG.14;

FIG. 18 is a timing chart showing another specific example of how theindividual circuits operate in a focus jump from a first-layer focuspoint to a 0th-layer focus point in the optical disc apparatus of FIG.14; and

FIG. 19 is a flowchart showing a specific example of a focus jumpcontrol algorithm of a microcomputer in the optical disc apparatus ofFIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, main symbols used in the drawing denote the following:

1 . . . Optical disc; 3 . . . Objective lens; 4 . . . Pickup; 7 . . .Signal processing circuit; 8 . . . Focus control circuit; 9 . . .Tracking control circuit; 12 . . . Differentiation circuit; 13 . . .Microcomputer; 14 . . . Low-pass filter; 15 . . . Previous value holdingcircuit; 16 . . . Elevation voltage value; 16 a . . . Elevation voltagevalue A; 16 b . . . Elevation voltage value B; 17 . . . Lowering voltagevalue; 17 a Lowering . . . voltage value A; 17 b . . . Lowering voltagevalue B; 18, 18 a, 18 b . . . Adder; 19 a-19 c, 19 e . . . Changeoverswitch; 19 d . . . On/off switch; 20 . . . Gain factor; 21 . . .Multiplier; 22, 22 a, 22 b . . . Signal level comparison circuit; 23 a .. . Threshold level A; 23 b . . . Threshold level B; 23 c . . .Threshold level C; 24 . . . Offset value; 25 . . . Value holdingcircuit; 26 . . . Deviation-from-layer preventing elevation voltage; 27. . . Deviation-from-layer preventing lowering voltage; 28 a . . .Threshold level A; 28 b . . . Threshold level B; 28 c . . . Thresholdlevel C; 28 d . . . Threshold level D; and 29 . . . Laser power controlcircuit.

Embodiments of the present invention will be hereinafter described withreference to the accompanying drawings.

FIG. 1 is a block diagram showing an optical disc apparatus and a focusjump method according to an embodiment of the invention. Referencesymbol 1 denotes a disc having two or more recording layers on one side;2 a, a clamper; 2 b, a turn table; 3 . . . an objective lens; 4, apickup; 5, a sled motor; 6, a spindle motor; 7, a signal processingcircuit; 8, a focus control circuit; 9, a tracking control circuit; 10,a sled control circuit; 11, a spindle control circuit; 12, adifferentiation circuit; 13, a microcomputer; 14, a low-pass filter(hereinafter referred to as “LPF”); 15, a previous value holdingcircuit; 16 a, an elevation voltage value A; 16 b, an elevation voltagevalue B; 17 a, a lowering voltage value A; 17 b, a lowering voltagevalue B; 18 a and 18 b, adders; 19 a-19 c, changeover switches; 19 d, anon/off switch; 19 e, a changeover switch; 20, a gain factor; 21, amultiplier; 22 a and 22 b, signal level comparison circuits; 23 a, athreshold level A; 23 b, a threshold level B; 23 c, a threshold level C;24, an offset value; and 25, a value holding circuit.

FIG. 7 shows, with the horizontal axis as a time axis, schematicwaveforms of an objective lens displacement, a focus error signal, afocus error differentiation signal, an objective lens drive signal, andan objective lens movement speed and operations of the switches 19 a-19d.

As shown in FIG. 1, the disc 1 that has been set on the turn table 2 bis fixed to the turn table 2 b by the damper 2 a. The disc 1 is rotatedas the spindle motor 6 is rotated.

To read out information on the disc 1, the microcomputer 13 supplies anemission control signal to a semiconductor laser that is incorporated inthe pickup 4.

FIG. 5 shows a configuration of the pickup 4 including the semiconductorlaser and an optical system as well as a configuration for focus errorsignal detection of the signal processing circuit 7. Reference numeral 1denotes the disc; 3, the objective lens; 51, a half prism; 52, thesemiconductor laser; 53, a condenser lens; 54, a photodetector; and 55,an error calculation unit (error amplifier).

In FIG. 1, a light beam emitted from the semiconductor laser 52 passesthrough the half prism 51, is given converging action by the objectivelens 3, and forms a beam spot on the disc 1. After being reflected bythe disc 1, the laser beam again passes through the objective lens 3, isreflected by the half prism 51, passes through the condenser lens 53,and forms a spot on the photodetector 54.

A specific structure of the photodetector 54 for detecting a focus errorsignal will be described below.

The photodetector 54 consists of four areas A-D in which areas on eachdiagonal line are paired with and connected electrically to each other.The photodetector 54 is located at such a position that when theobjective lens 3 is focused on the disc 1, the beam spot on thephotodetector 54 assumes a circle and hence an output of the erroramplifier 55 obtained by amplifying the difference between the sums ofoutputs of the diagonal areas of the photodetector 54 becomes zero. Ifthe disc 1 is deviated vertically from the focus position of theobjective lens 3, the beam spot on the photodetector 54 assumes anellipse that is longer in the vertical direction or horizontaldirection. Based on this phenomenon, a focus error signal (FE signal) asshown in FIG. 6 that reflects the amount and direction of a deviationfrom the focus position is obtained from the error amplifier 55 (what iscalled the astigmatism method).

In FIG. 6, the horizontal axis represents the distance between theobjective lens 3 and the disc 1 and the vertical axis represents thelevel of a focus error signal. The focus error signal has a feature thatits S-shaped curve crosses the zero level when the objective lens 3 isfocused on a disc recording layer. The S-shaped curve may have oppositepolarities depending on how the outputs of the photodetector 54 areconnected to the error amplifier 55. In the case of a system in whichthe S-shaped curve has opposite polarities, naturally it is proper toconsider that the relationship between the signal level and the discdisplacement is opposite.

The focus error signal generated by the error amplifier 55 is suppliedto the focus control circuit 8 (see FIG. 1), which generates, by using adelay compensator, a lead compensator, etc., and outputs a drive signalfor an actuator (not shown) for moving the objective lens 3, to enable afeedback control in the vicinity of the zero-cross point of an S-shapedcurve of the focus error signal. The output signal is supplied to thechangeover switch 19 b. In a steady state, the changeover switch 19 b isswitched to the G-side according to an instruction from themicrocomputer 13 and thereby supplies a focus control signal to thepickup 4 as a drive signal. The vertical position of the objective lens3 is controlled according to this drive signal and a feedback loop focuscontrol is realized, whereby the disc 1 always stays at a focusposition.

On the other hand, a tracking error signal (TE signal) generated by thesignal processing circuit 7 is supplied to the tracking error controlcircuit 9, which generates a drive signal for moving the objective lens3 in the tracking direction by a feedback control. This drive signal issupplied to the pickup 4. The position of the objective lens 3 iscontrolled in the tracking direction according to the drive signal thatis supplied to the inside of the pickup 4, and a feedback loop trackingcontrol is thereby realized. The beam spot is always located on pitsthat are formed in a recording layer of the disc 1. The drive signalthat is output from the tracking control circuit 9 is also supplied tothe sled control circuit 10, which generates a drive signal forcontrolling the sled motor 5 in accordance with the deviation of theobjective lens 3 in the tracking direction. The sled motor 5 is drivenaccording to the drive signal received, whereby a sled (base) of thepickup 4, that is, the pickup 4 itself, is moved.

Further, the signal processing circuit 7 supplies the spindle controlcircuit 11 with rotation cycle information that is read from the disc 1.The spindle control circuit 11 generates a signal for driving thespindle motor 6 based on the received rotation cycle information andsupplies it to the spindle motor 6.

In a steady state, the focus, tracking, spindle, and sled controls areperformed in the above-described manners with the objective lens 3focused on a recording layer.

Where the disc 1 is a DVD having two recording layers on one side, theremay occur a case that it is necessary to switch from a focus pointcorresponding to a recording layer at which the beam is focused toanother focus point corresponding to the other recording layer. Adescription will be made below of an exemplary case that the objectivelens 3 is located at such a position as to be focused on the 0threcording layer and it is desired to move the focus position of theobjective lens 3 to the first recording layer (i.e., it is desired tojump from a focus point corresponding to the lower (0th) recording layerto a focus point corresponding to the upper (first) recording layer).

In a steady state, a drive signal for driving the objective lens 3 thatis output from the focus control circuit 8 being in a state that a focuspoint corresponding to the 0th recording layer is established issupplied to the on/off switch 19 d. Since the on/off switch 19 d isclosed in this steady state, the drive signal is supplied to theprevious value holding circuit 15 as it is. The previous value holdingcircuit 15 continues to hold, as it is, a value that has been held sofar and supplies it to the LPF 14 until the value of the drive signalchanges. The LPF 14 has such a frequency characteristic as to rejecthigh-frequency components (noise components) of the signal for drivingthe objective lens 3 and not to reject low-frequency components such asa surface vibration component that is caused by rotation of the disc 1having a warp or the like. As such, the LPF 14 rejects mainly noisecomponents from the drive signal and supplies a resulting signal to theadder 18 a. In a steady state, the components to the LPF 14 alwaysoperate.

To cause a focus jump to a focus point corresponding to the firstrecording layer, the microcomputer 13 sets, in drive voltage generationcircuits, initial values of drive voltages necessary for the focus jump,that is, a constant elevation voltage value A (16 a) and a constantelevation voltage value B (16 b) that are acceleration voltage valuesand a constant lowering voltage value A (17 a) and a constant loweringvoltage value B (17 b) that are deceleration voltage values necessary todecelerate and stop, after acceleration, the objective lens 3 at aposition corresponding to a first-layer focus point. Further, themicrocomputer 13 sets, in their setting circuits, initial values of athreshold level A (23 a), a threshold level B (23 b), a threshold valueC (23 c), an offset value 24, and a gain factor 20. After setting thoseinitial values, the microcomputer 13 switches the changeover switches 19b and 19 c to the H-side and the B-side, respectively, and opens theon/off switch 19 d. As a result of the switching of the changeoverswitch 19 b and the on/off switch 19 d, the feedback loop by which theobjective lens 3 has been controlled so far is made an open loop and thefeedback control is stopped.

Then, the microcomputer 13 issues, to the changeover switch 19 a, aninstruction to switch to the C-side, whereby the elevation voltage valueA (16 a) is supplied to the adder 18 a. The adder 18 a adds theelevation voltage value A (16 a) to a signal that is free ofhigh-frequency noise components (rejected by the LPF 14), and supplies aresulting addition signal to the changeover switch 19 c. Since thechangeover switch 19 c is switched to the B-side, the addition signal issupplied from the changeover switch 19 c to the changeover switch 19 bas it is. Since the changeover switch 19 b is switched to the H-side,the addition signal is supplied to the pickup 4 via the changeoverswitch 19 b. Since the elevation voltage value A (16 a) is applied tothe actuator, the objective lens 3 starts to go up.

In FIG. 7, time point A is a start point of the focus jump. Theelevation voltage value A (16 a; also denoted by “Vup1” in FIG. 7) isapplied, as it is, as an objective lens drive signal, to the actuatorfor driving the objective lens 3.

Referring to FIG. 1, a focus error signal that is output from the signalprocessing circuit 7 is supplied to the differentiation circuit 12. Thedifferentiation circuit 12 differentiates the received focus errorsignal. The differentiation circuit 12 may be a high-pass filter (HPF)that performs differentiation with respect to time in a prescribed band.

The focus error signal that is output from the signal processing circuit7 is also supplied to the comparison circuit 22 a, the adder 18 b, andthe value holding circuit 25.

FIG. 7 shows a focus error signal and a focus error differentiationsignal (hereinafter abbreviated as “differentiation signal”) that occurwhen a focus jump from a focus point corresponding to the 0th recordinglayer to a focus point corresponding to the first recording layer isperformed. Those signals will be described below in detail for each ofsections that are defined by time points A-I.

When the objective lens 3 goes up after the focus jump was started attime point A, the focus error signal rises gradually from a level closeto the middle level until time point B. In this section from time pointA to B, the differentiation signal rises gradually from a level close tothe middle level, reaches a maximum value, then gradually decreases, andfinally returns to the middle level (zero) at time point B when thefocus error signal has a maximum value. As the objective lens 3 goes upfurther, at time point D the focus position enters the interlayer regionbetween the 0th recording layer and the first recording layer. The focuserror signal decreases gradually from the maximum value and reaches themiddle level (zero). In the section from time point B to D, thedifferentiation signal decreases from the middle level (zero), reaches aminimum value, then increases gradually, and finally reaches the middlelevel (zero) again. In the section from time point D to E thatcorresponds to the interlayer region, both of the focus error signal andthe differentiation signal are at the middle level (zero). As theobjective lens 3 goes up further, the focus position enters the firstlayer region and hence the focus error signal falls gradually from alevel close to the middle level until time point G. In the section fromtime point E to G, the difference signal falls gradually from a levelclose to the middle level, reaches a minimum value, then graduallyincreases, and finally reaches the middle level (zero) at time point Gwhen the focus error signal has a minimum value. As the objective lens 3goes up further, a first-layer focus point is established at time pointI. The focus error signal gradually increases from the minimum value andreaches the middle level (zero). In the section from time point G to I,the differentiation signal increases from the middle level (zero),reaches a maximum value, then decreases gradually, and finally reachesthe middle level (zero). At time point I when the first-layer focuspoint is established, the focus error signal and the differentiationsignal reach the respective middle levels (zero).

By using the differentiation signal, more specifically, by detecting atime point (zero-cross point) when the differentiation signal crossesthe middle level (zero), a position of the objective lens 3corresponding to time point B can be detected easily and reliably.Although time point B can also be detected by monitoring the level ofthe focus error signal, the detection is not reliable because theamplitude of the focus error signal varies depending on the disc, forexample, and hence is not uniform.

Therefore, the differentiation signal that is output from thedifferentiation circuit 12 is supplied to the microcomputer 13. Themicrocomputer 13 detects that the objective lens 3 has passed a positioncorresponding to time point B by detecting a time point (zero-crosspoint) when the differentiation signal received reaches the middle level(zero).

When detecting, first time, the position corresponding to time point B,the microcomputer 13 issues, to the changeover switch 19 e, aninstruction to switch to the K-side, whereby the value of the thresholdlevel A (23 a) is supplied to the comparison circuit 22 a. Thecomparison circuit 22 a compares the focus error signal that is suppliedfrom the signal processing circuit 7 with the threshold level A (23 a)and supplies a comparison result to the microcomputer 13. As theobjective lens 3 goes up further, the level of the focus error signalbecomes lower than the threshold level A (23 a) at time point C,whereupon the comparison circuit 22 a supplies a comparison detectionsignal to the microcomputer 13. When receiving the comparison detectionsignal, the microcomputer 13 switches the changeover switch 19 a to theD-side, whereby the elevation voltage value B (16 b) is supplied to theadder 18 a.

The adder 18 a adds the elevation voltage value B (16 b) to a signalthat is free of high-frequency noise components (rejected by the LPF14), and supplies a resulting addition signal to the changeover switch19 c. Since the changeover switch 19 c is switched to the B-side, theaddition signal is supplied from the changeover switch 19 c to thechangeover switch 19 b as it is. Since the changeover switch 19 b isswitched to the H-side, the addition signal is supplied to the pickup 4via the changeover switch 19 b. Since the elevation voltage value B (16b) is applied to the actuator, the objective lens 3 continues to go up.

As shown in FIG. 7, after the elevation voltage value switching timepoint C, the elevation voltage value B (16 b; also denoted by “Vup2”) issupplied, as it is, to the actuator for driving the objective lens 3.The elevation voltage value B (16 b) is set smaller than the elevationvoltage value A (16 a). Therefore, the elevation speed of the objectivelens 3 is lower than when the elevation voltage value A (16 a) wasapplied to the actuator. After time point C has been passed, themicrocomputer 13 issues, to the changeover switch 19 e, an instructionto switch to the L-side. The changeover switch 19 e supplies thethreshold level B (23 b) to the signal level comparison circuit 22 a.The signal level comparison circuit 22 a compares the level of the focuserror signal that is supplied from the signal processing circuit 7 withthe threshold level B (23 b). When the level of the focus error signalbecomes lower than the threshold level B (23 b) (time point F in FIG.7), the signal level comparison circuit 22 a supplies a signal to thateffect to the microcomputer 13. When detecting that timepoint F has beenpassed, to apply a voltage value for decelerating the objective lens 3that has continued to go up, the microcomputer 13 issues, to thechangeover switch 19 c, an instruction to switch to the A-side.

At this time, the focus error signal that is output from the signalprocessing circuit 7 is supplied to the differentiation circuit 12. Adifferentiated version of the focus error signal generated by thedifferentiation circuit 12 is supplied to the multiplier 21. Themultiplier 21 supplies the changeover switch 19 c with a result obtainedby multiplying the output (differentiation signal) of thedifferentiation circuit 12 by the gain factor 20. Since the changeoverswitch 19 c is switched to the A-side, the multiplication result issupplied to the changeover switch 19 b as it is. The signal obtained bymultiplying the differentiation signal by the gain factor 20 issupplied, as a deceleration signal, to the actuator via the changeoverswitch 19 b.

In the section from time point F to G, the focus error signal reflectsthe displacement of the objective lens 3 (it decreases monotonously).Since in general speed is obtained by differentiating displacement withrespect to time, a signal obtained by differentiating the focus errorsignal represents the speed of the objective lens 3. For example, if ahigh elevation voltage has been applied to the actuator and hence anelevation speed of the objective lens 3 when switching is made to thedeceleration voltage is high, the focus error signal falls steeply fromtime point F to G. Therefore, a signal obtained by differentiating sucha focus error signal has a large value, that is, the decelerationvoltage has a large value, which means that the force of decreasing theelevation speed of the objective lens 3 is strong. Conversely, if a lowelevation voltage has been applied to the actuator and hence anelevation speed of the objective lens 3 when switching is made to thedeceleration voltage is low, the focus error signal falls gently fromtime point F to G. Therefore, a signal obtained by differentiating sucha focus error signal has a small value, that is, the decelerationvoltage has a small value, which means that the force of decreasing theelevation speed of the objective lens 3 is weak.

A deceleration voltage value corresponding to an elevation speed of theobjective lens 3 can be obtained and the elevation speed of theobjective lens 3 can be decreased by using a signal obtained bydifferentiating the focus error signal from time F to G in theabove-described manner. The gain factor 20 is used for adjusting theamplitude of the deceleration voltage that is obtained bydifferentiating the focus error signal.

The objective lens 3 continues to go up even after the application ofthe deceleration voltage to the actuator. After time point F has beenpassed, the microcomputer 13 monitors the focus error signal and detectsits minimum value.

A method for detecting a minimum value will be described below.

A focus error signal that is output from the signal processing circuit 7is supplied to the adder 18 b and the value holding circuit 25. Theadder 18 b adds together the offset value 24 and the focus error signalthat is supplied from the signal processing circuit 7, and supplies aresulting signal to the comparison circuit 22 b. The offset value isused for preventing erroneous detection of a minimum value when thefocus error signal is influenced by noise or the like. According to aninstruction from the microcomputer 13, the comparison circuit 22 bcompares the offset-value-added output of the adder 18 b with the outputof the value holding circuit 25. If the output value of the adder 18 bis smaller than the output value of the value holding circuit 25, thecomparison circuit 22 b supplies a comparison result signal to the valueholding circuit 25. In response to the comparison result signal, thevalue holding circuit 25 updates the value that has been held so far tothe value of the focus error signal.

FIG. 11 schematically shows how a minimum value is detected.

In FIG. 11, the solid line represents a focus error signal and a dottedline represents a signal obtained by adding an offset value to the focuserror signal. Where no offset value is added to the focus error signal,that is, the offset value is zero, the solid line representing the focuserror signal coincides with the dotted line representing theoffset-value-added signal. In this case, first, a value of point 1 isheld by the value holding circuit 25 as a minimum value. Then, thisminimum value is compared with a value of point 2. Since the value ofpoint 2 is smaller than the minimum value, the former is employed as anew minimum value. Then, the new minimum value is compared with a valueof point 3. Since the value of point 3 is larger than the minimum value,the minimum value is not updated; the value of point 2 is finally judgedas a minimum value of the focus error signal.

A description will now be made of a case where an offset signal is addedto a focus error signal.

In the case of detecting a minimum value, the offset value is made anegative value. As a result, a signal obtained by adding the offsetvalue to the original signal (in this case, the focus error signal) hassmaller values than the original signal. To detect a minimum value,first, a value of point 1 is held by the value holding circuit 25 as aminimum value. Then, the minimum value is compared with a value of pointB obtained by adding the offset value to a value of point 2. Since thevalue of point B is smaller than the minimum value, the value of point 2is employed as a new minimum value. Then, the new minimum value iscompared with a value of point C that is obtained by adding the offsetvalue to a value of point 3. Since the value of point C is smaller thanthe minimum value, the value of point 3 is employed as a new minimumvalue. Then, the new minimum value is compared with a value of point Dthat is obtained by adding the offset value to a value of point 4. Sincethe value of point D is smaller than the minimum value, the value ofpoint 4 is employed as a new minimum value. Then, the new minimum valueis compared with a value of point E that is obtained by adding theoffset value to a value of point 5. Since the value of point E issmaller than the minimum value, the value of point 5 is employed as anew minimum value. Then, the new minimum value is compared with a valueof point F that is obtained by adding the offset value to a value ofpoint 6. Since the value of point F is smaller than the minimum value,the value of point 6 is employed as a new minimum value. Then, the newminimum value is compared with a value of point G that is obtained byadding the offset value to a value of point 7. Since the value of pointG is larger than the minimum value, the minimum value is not updated;the value of point 6 is finally judged as a minimum value of the focuserror signal.

In the example of FIG. 11, the actual minimum value of the focus errorsignal is the value of point 5. Where the offset value is added to thefocus error signal, the value of point 6 is finally judged as a minimumvalue of the focus error signal. However, if no offset signal is addedto the focus error signal, the value of point 2 is finally judged as aminimum value of the focus error signal.

By using a signal obtained by adding an offset value to a focus errorsignal, a minimum value can be detected without being influenced bynoise or disturbance whose amplitude is smaller than the offset value.

The microcomputer 13 judges that time point G has been passed, at a timepoint when the comparison result signal comes to indicate that theminimum value of the focus error signal has not been updated. Whendetecting the passage of time point G, to stop stably the objective lens3 that is about to make a transition from ascent to descent and toestablish a first-layer focus point (time point I in FIG. 7), themicrocomputer 13 issues, to the changeover switches 19 a and 19 c,instructions to switch to the E-side and the B-side, respectively.Switched to the E-side, the changeover switch 19 a outputs the loweringvoltage value A (17 a). The adder 18 a adds together the loweringvoltage value A (17 a) and a signal that is free of high-frequency noisecomponents (rejected by the LPF 14), and supplies a resulting additionsignal to the changeover switch 19 c. Since the changeover switch 19 cis switched to the B-side, the addition signal is supplied from thechangeover switch 19 c to the changeover switch 19 b as it is. Since thechangeover switch 19 b is switched to the H-side, the addition signal issupplied, via the changeover switch 19 b, to the pickup 4, where it isapplied to the actuator for driving the objective lens 3. Because of theapplication of the lowering voltage value A (17 a) to the actuator, theelevation speed of the objective lens 3 is decreased and the objectivelens 3 stops ascending.

After the objective lens 3 has passed the position corresponding to timepoint G, the microcomputer 13 issues, to the changeover switch 19 e, toswitch to the M-side, whereby the threshold level C (23 c) is suppliedto the comparison circuit 22 a. The comparison circuit 22 a compares thefocus error signal that is supplied from the signal processing circuit 7with the threshold level C (23 c), and supplies a comparison result tothe microcomputer 13. Further, after the objective lens 3 has passed theposition corresponding to time point G, the microcomputer 13 monitorsthe focus error signal and detects its maximum value.

A method for detecting a maximum value will be described below.

A focus error signal that is output from the signal processing circuit 7is supplied to the adder 18 b and the value holding circuit 25. Theadder 18 b adds together the offset value 24 and the focus error signalthat is supplied from the signal processing circuit 7, and supplies aresulting signal to the comparison circuit 22 b. According to aninstruction from the microcomputer 13, the comparison circuit 22 bcompares the output of the adder 18 b with the output of the valueholding circuit 25. If the output value of the adder 18 b is larger thanthe output value of the value holding circuit 25, the comparison circuit22 b supplies a comparison result signal to the value holding circuit25. In response to the comparison result signal, the value holdingcircuit 25 updates the value that has been held so far to the value ofthe focus error signal.

In this manner, the maximum value of values, input so far, of the focuserror signal is always held by the value holding circuit 25.

FIG. 12 schematically shows how a maximum value is detected by the valueholding circuit 25. In FIG. 12, the solid line represents a focus errorsignal and a dotted line represents a signal obtained by adding anoffset value to the focus error signal.

Where no offset value is added to the focus error signal, that is, theoffset value is zero, the solid line representing the focus error signalcoincides with the dotted line representing the offset-value-addedsignal. In this case, first, a value of point 1 is held by the valueholding circuit 25 as a maximum value. Then, this maximum value iscompared with a value of point 2. Since the value of point 2 is largerthan the maximum value, the letter is employed as a new maximum value.Then, the new maximum value is compared with a value of point 3. Sincethe value of point 3 is larger than the maximum value, the latter isemployed as a new maximum value. Then, the new maximum value is comparedwith a value of point 4. Since the maximum value (i.e., the value ofpoint 3) is larger than the value of point 4, the maximum value is notupdated; the value of point 3 is finally held as a maximum value of thefocus error signal.

Next, a description will be made of a case where an offset signal isadded to a focus error signal.

In the case of detecting a maximum value, the offset value is made apositive value. As a result, a signal obtained by adding the offsetvalue to the original signal (in this case, the focus error signal) haslarger values than the original signal. To detect a maximum value,first, a value of point 1 is held by the value holding circuit 25 as amaximum value. Then, the maximum value is compared with a value of pointB obtained by adding the offset value to a value of point 2. Since thevalue of point B is larger than the maximum value, the value of point 2is employed as a new maximum value. Then, the new maximum value iscompared with a value of point C that is obtained by adding the offsetvalue to a value of point 3. Since the value of point C is larger thanthe maximum value, the value of point 3 is employed as a new maximumvalue. Then, the new maximum value is compared with a value of point Dthat is obtained by adding the offset value to a value of point 4. Sincethe value of point D is larger than the maximum value, the value ofpoint 4 is employed as a new maximum value. Then, the new maximum valueis compared with a value of point E that is obtained by adding theoffset value to a value of point 5. Since the value of point E is largerthan the maximum value, the value of point 5 is employed as a newmaximum value. Then, the new maximum value is compared with a value ofpoint F that is obtained by adding the offset value to a value of point6. Since the value of point F is smaller than the maximum value, themaximum value is not updated; the value of point 5 is finally judged asa maximum value of the focus error signal.

As described above, in the example of FIG. 12, if no offset signal isadded to the focus error signal, the value of point 3 is finally judgedas a maximum value of the focus error signal. In contrast, in the casewhere the offset value is added to the focus error signal, among thevalues of point 1 to point 8 the value of point 5 is judged as a maximumvalue. Therefore, by using a signal obtained by adding an offset valueto an original signal, a maximum value can be detected without beinginfluenced by noise or disturbance whose amplitude is smaller than theoffset value.

The microcomputer 13 judges that the objective lens 3 has made atransition from ascent to descent, at a time point when the comparisonresult signal comes to indicate that the maximum value of the focuserror signal has not been updated. As the objective lens 3 continues toascend, the level of the focus error signal exceeds the threshold levelC (23 c) at time point H. The comparison circuit 22 a supplies acomparison detection signal to that effect to the microcomputer 13. Inresponse to this comparison detection signal, the microcomputer 13switches the changeover switch 19 b to the G-side. The speed of theobjective lens 3 is approximately equal to zero when it is focused at aposition close to the first recording layer. Therefore, by performing afeedback loop focus control using the focus error signal, the objectivelens 3 can be pulled into a position where it is focused on the firstrecording layer.

FIG. 7 shows a case that the elevation voltage and the lowering voltageare well balanced. In this case, the elevation speed of the objectivelens 3 becomes zero when it is focused at a position close to the firstlayer recording layer, and the objective lens 3 is pulled into aposition where it is focused on the first recording layer without makinga transition to descent.

FIG. 8 similarly shows a case that the objective lens 3 is focused onthe 0th recording layer and it is desired to move the focus position ofthe objective lens 3 to the first layer recording layer (i.e., it isdesired to jump from a focus point corresponding to the lower (0th)recording layer to that corresponding to the upper (first) recordinglayer) and that the deceleration voltage is too high relative to theacceleration voltage and hence, if no proper measure were taken, theobjective lens 3 would start to descend before reaching a position whereit is focused on the first recording layer.

Referring to FIG. 8, the controls from time point A to G are the same asdescribed above. That is, the elevation voltage value A (16 a) isapplied first and then the elevation voltage value B (16 b) is applied.After the focus position has passed through the interlayer regionbetween the 0th recording layer and the first recording layer, a speedcontrol is performed by using a signal obtained by the differentiationcircuit 12's differentiating the focus error signal that is suppliedfrom the signal processing circuit 7. The objective lens 3 continues toascend until time point G. Because of the speed control, the elevationspeed becomes zero at a time point close to time point G. The objectivelens 3 starts to descend because the lowering voltage value A (17 a) isapplied to the actuator after passage of time point G. Although thefocus jump from the 0th-layer focus point to the first-layer focus pointis intended, a return to the 0th-layer focus point is started (timepoint J).

The focus jump would fail if the above operation continued. To avoidsuch a failure, the operation of returning to the 0th-layer focus pointis detected by the above-described maximum value detection by themicrocomputer 13. When detecting the returning operation, themicrocomputer 13 switches the changeover switch 19 b to the G-sidewithout waiting for an event that the level of the focus error signalexceeds the threshold level C (23 c). At this time point, the focusposition of the objective lens 3 is a little distant from the firstrecording layer. However, since the speed of the objective lens 3 isapproximately zero, the objective lens 3 is located in such a regionthat a feedback loop focus control using the focus error signal can beperformed successfully and hence the objective lens 3 can be pulled intoa position where it is focused on the first recording layer.

Next, referring to FIG. 9, a description will be made of a case that theobjective lens 3 is focused on the first recording layer and it isintended to move the focus position of the objective lens 3 to the 0threcording layer (i.e., it is intended to jump from a focus pointcorresponding to the upper (first) recording layer to that correspondingto the lower (0th) recording layer).

Currently the apparatus is in a steady state. An objective lens 3 drivesignal that is output from the focus control circuit 8 in such a statethat the first-layer focus point is established is supplied to theon/off switch 19 d. Since in this steady state the on/off switch 19 d isclosed, the drive signal is supplied to the previous value holdingcircuit 15 as it is. The previous value holding circuit 15 continues tohold the value held so far and supplies it to the LPF 14 until the valueof the received drive signal varies. The LPF 14 has such a frequencycharacteristic as to reject high-frequency components (noise components)of the objective lens 3 drive signal and not to reject low-frequencycomponents such as a surface vibration component that is caused byrotation of the disc 1 having a warp or the like. As such, the LPF 14rejects mainly noise components from the drive signal and supplies aresulting signal to the adder 18 a. In a steady state, the components tothe LPF 14 always operate.

To cause a focus jump to a focus point corresponding to the 0threcording layer, the microcomputer 13 sets initial values of a constantlowering voltage value A (17 a) and a constant lowering voltage value B(17 b) that are acceleration voltage values necessary for the interlayermovement, a constant elevation voltage value A (16 a) a threshold levelA (23 a), a threshold level B (23 b), a threshold value C (23 c), anoffset value 24, and a gain factor 20.

After setting those initial values, the microcomputer 13 switches thechangeover switches 19 b and 19 c to the H-side and the B-side,respectively, and opens the on/off switch 19 d. As a result of theswitching of the changeover switch 19 b and the on/off switch 19 d, thefeedback loop by which the objective lens 3 has been controlled so faris made an open loop and the feedback control is stopped. Then, themicrocomputer 13 issues, to the changeover switch 19 a, an instructionto switch to the E-side, whereby the lowering voltage value A (17 a) issupplied to the adder 18 a. The adder 18 a adds the lowering voltagevalue A (17 a) to a signal that is free of high-frequency noisecomponents (rejected by the LPF 14), and supplies a resulting additionsignal to the changeover switch 19 c. Since the changeover switch 19 cis switched to the B-side, the addition signal is supplied from thechangeover switch 19 c to the changeover switch 19 b as it is. Since thechangeover switch 19 b is switched to the H-side, the addition signal issupplied to the pickup 4 via the changeover switch 19 b. Since thelowering voltage value A (17 a) is applied to the actuator, theobjective lens 3 starts to go down.

In FIG. 9, time point A is a start point of the focus jump. The loweringvoltage value A (17 a; also denoted by “Vdw1” in FIG. 9) is applied, asit is, as an objective lens drive signal, to the actuator for drivingthe objective lens 3.

A focus error signal that is output from the signal processing circuit 7is supplied to the differentiation circuit 12. The differentiationcircuit 12 differentiates the received focus error signal. Thedifferentiation circuit 12 may be a high-pass filter (HPF) that performsdifferentiation with respect to time in a prescribed band.

The focus error signal that is output from the signal processing circuit7 is also supplied to the comparison circuit 22 a, the adder 18 b, andthe value holding circuit 25.

FIG. 9 shows a focus error signal and a focus error differentiationsignal (hereinafter abbreviated as “differentiation signal”) that occurwhen a focus jump from a focus point corresponding to the firstrecording layer to a focus point corresponding to the 0th recordinglayer is performed. Those signals will be described below in detail foreach of sections that are defined by time points A-I.

When the objective lens 3 goes down after the focus jump was started attime point A, the focus error signal falls gradually from a level closeto the middle level until time point B. In this section from time pointA to B, the differentiation signal falls gradually from a level close tothe middle level, reaches a minimum value, then gradually increases, andfinally returns to the middle level (zero) at time point B when thefocus error signal has a minimum value. As the objective lens 3 goesdown further, at time point D the focus position enters the interlayerregion between the first recording layer and the 0th recording layer.The focus error signal increases gradually from the minimum value andreaches the middle level (zero). In the section from time point B to D,the differentiation signal increases from the middle level (zero),reaches a maximum value, then decreases gradually, and finally reachesthe middle level (zero) again. In the section from time point D to Ethat corresponds to the interlayer region, both of the focus errorsignal and the differentiation signal are at the middle level (zero). Asthe objective lens 3 goes down further, the focus position enters the0th layer region and hence the focus error signal rises gradually from alevel close to the middle level until time point G. In the section fromtime point E to G, the difference signal rises gradually from a levelclose to the middle level, reaches a maximum value, then graduallydecreases, and finally reaches the middle level (zero) at time point Gwhen the focus error signal has a maximum value. As the objective lens 3goes down further, a 0th-layer focus point is established at time pointI. The focus error signal gradually decreases from the maximum value andreaches the middle level (zero). In the section from time point G to I,the differentiation signal decreases from the middle level (zero),reaches a minimum value, then increases gradually, and finally reachesthe middle level (zero). At time point I when the 0th-layer focus pointis established, the focus error signal and the differentiation signalreach the respective middle levels (zero).

By using the differentiation signal, more specifically, by detecting atime point (zero-cross point) when the differentiation signal crossesthe middle level (zero), a position of the objective lens 3corresponding to time point B can be detected easily and reliably.Although time point B can also be detected by monitoring the level ofthe focus error signal, the detection is not reliable because theamplitude of the focus error signal varies depending on the disc, forexample, and hence is not uniform.

The differentiation signal that is output from the differentiationcircuit 12 is supplied to the microcomputer 13. The microcomputer 13detects that the objective lens 3 has passed a position corresponding totime point B by detecting a time point (zero-cross point) when thedifferentiation signal received reaches the middle level (zero). Whendetecting, first time, the position corresponding to time point B, themicrocomputer 13 issues, to the changeover switch 19 e, an instructionto switch to the K-side, whereby the threshold level A (23 a) issupplied to the comparison circuit 22 a. The comparison circuit 22 acompares the focus error signal that is supplied from the signalprocessing circuit 7 with the threshold level A (23 a) and supplies acomparison result to the microcomputer 13.

As the objective lens 3 goes down further, the level of the focus errorsignal becomes higher than the threshold level A (23 a) at time point C,whereupon the comparison circuit 22 a supplies a comparison detectionsignal to the microcomputer 13. When receiving the comparison detectionsignal, the microcomputer 13 switches the changeover switch 19 a to theF-side, whereby the lowering voltage value B (17 b) is supplied to theadder 18 a. The adder 18 adds the lowering voltage value B (17 b) to asignal that is free of high-frequency noise components (rejected by theLPF 14), and supplies a resulting addition signal to the changeoverswitch 19 c. Since the changeover switch 19 c is switched to the B-side,the addition signal is supplied from the changeover switch 19 c to thechangeover switch 19 b as it is. Since the changeover switch 19 b isswitched to the H-side, the addition signal is supplied to the pickup 4via the changeover switch 19 b. Since the lowering voltage value B (17b) is applied to the actuator, the objective lens 3 continues to godown.

As shown in FIG. 9, after the lowering voltage value switching timepoint C, the lowering voltage value B (17 b; also denoted by “Vdw2”) issupplied, as it is, to the actuator for driving the objective lens 3.The lowering voltage value B (17 b) is set smaller than the loweringvoltage value A (17 a). Therefore, the lowering speed of the objectivelens 3 is lower than when the lowering voltage value A (17 a) wasapplied to the actuator.

After time point C has been passed, the microcomputer 13 issues, to thechangeover switch 19 e, an instruction to switch to the L-side. Thechangeover switch 19 e supplies the threshold level B (23 b) to thesignal level comparison circuit 22 a. The signal level comparisoncircuit 22 a compares the level of the focus error signal that issupplied from the signal processing circuit 7 with the threshold level B(23 b). When the level of the focus error signal becomes higher than thethreshold level B (23 b) (time point F in FIG. 9), the signal levelcomparison circuit 22 a supplies a signal to that effect to themicrocomputer 13.

When detecting that time point F has been passed, to apply a voltagevalue for decelerating the objective lens 3 that has continued to godown, the microcomputer 13 issues, to the changeover switch 19 c, aninstruction to switch to the A-side. At this time, the focus errorsignal that is output from the signal processing circuit 7 is suppliedto the differentiation circuit 12. A differentiated version of the focuserror signal generated by the differentiation circuit 12 is supplied tothe multiplier 21. The multiplier 21 supplies the changeover switch 19 cwith a result obtained by multiplying the differentiation signal by thegain factor 20. Since the changeover switch C 19 c is switched to theA-side, the multiplication result is supplied to the changeover switch19 b as it is. The signal obtained by multiplying the differentiationsignal by the gain factor 20 is supplied, as a deceleration signal, tothe actuator via the changeover switch 19 b.

In the section from time point F to G, the focus error signal reflectsthe displacement of the objective lens 3 (it increases monotonously).Since in general speed is obtained by differentiating displacement withrespect to time, a signal obtained by differentiating the focus errorsignal represents the movement speed of the objective lens 3. Forexample, if a high lowering voltage has been applied to the actuator andhence a lowering speed of the objective lens 3 when switching is made tothe deceleration voltage is high, the focus error signal rises steeplyfrom time point F to G. Therefore, a signal obtained by differentiatingsuch a focus error signal has a large value, that is, the decelerationvoltage has a large value, which means that the force of decreasing thelowering speed of the objective lens 3 is strong. Conversely, if a lowlowering voltage has been applied to the actuator and hence a loweringspeed of the objective lens 3 when switching is made to the decelerationvoltage is low, the focus error signal rises gently from time point F toG. Therefore, a signal obtained by differentiating such a focus errorsignal has a small value, that is, the deceleration voltage has a smallvalue, which means that the force of decreasing the lowering speed ofthe objective lens 3 is weak.

A deceleration voltage value corresponding to a lowering speed of theobjective lens 3 can be obtained and the lowering speed of the objectivelens 3 can be decreased by using a signal obtained by differentiatingthe focus error signal from time F to G in the above-described manner.

The gain factor 20 is used for adjusting the amplitude of thedeceleration voltage that is obtained by differentiating the focus errorsignal. The objective lens 3 continues to go down even after theapplication of the deceleration voltage to the actuator. After timepoint F has been passed, the microcomputer 13 monitors the focus errorsignal and detects its maximum value. The above-described method fordetecting a maximum value is used here.

That is, the focus error signal that is output from the signalprocessing circuit 7 is supplied to the adder 18 b and the value holdingcircuit 25. The adder 18 b adds together the offset value 24 and thefocus error signal that is supplied from the signal processing circuit7, and supplies a resulting signal to the comparison circuit 22 b.According to an instruction from the microcomputer 13, the comparisoncircuit 22 b compares the output of the adder 18 b with the output ofthe value holding circuit 25. If the output value of the adder 18 b islarger than the output value of the value holding circuit 25, thecomparison circuit 22 b supplies a comparison result signal to the valueholding circuit 25. In response to the comparison result signal, thevalue holding circuit 25 updates the value that has been held so far tothe value of the focus error signal. In this manner, the maximum valueof values, input so far, of the focus error signal is always held by thevalue holding circuit 25.

The microcomputer 13 judges that time point G has been passed, at a timepoint when the comparison result signal comes to indicate that themaximum value of the focus error signal has not been updated. Whendetecting the passage of time point G, to stop stably the objective lens3 that is about to make a transition from descent to ascent and toestablish a 0th-layer focus point (time point I in FIG. 9), themicrocomputer 13 issues, to the changeover switches 19 a and 19 c,instructions to switch to the C-side and the B-side, respectively.Switched to the C-side, the changeover switch 19 a outputs the elevationvoltage value A (16 a). The adder 18 a adds together the elevationvoltage value A (16 a) and a signal that is free of high-frequency noisecomponents (rejected by the LPF 14), and supplies a resulting additionsignal to the changeover switch 19 c. Since the changeover switch 19 cis switched to the B-side, the addition signal is supplied from thechangeover switch 19 c to the changeover switch 19 b as it is. Since thechangeover switch 19 b is switched to the H-side, the addition signal issupplied, via the changeover switch 19 b, to the pickup 4, where it isapplied to the actuator for driving the objective lens 3. Because of theapplication of the elevation voltage value A (16 a) to the actuator, theelevation speed of the objective lens 3 is decreased and the objectivelens 3 stops descending.

After judging that time point G has been passed, the microcomputer 13issues, to the changeover switch 19 e, to switch to the M-side, wherebythe threshold level C (23 c) is supplied to the comparison circuit 22 a.The comparison circuit 22 a compares the focus error signal that issupplied from the signal processing circuit 7 with the threshold level C(23 c), and supplies a comparison result to the microcomputer 13.

Further, after judging that time point G has been passed, themicrocomputer 13 monitors the focus error signal and detects its maximumvalue. The above-described method for detecting a maximum value is usedhere.

That is, the focus error signal that is output from the signalprocessing circuit 7 is supplied to the adder 18 b and the value holdingcircuit 25. The adder 18 b adds together the offset value 24 and thefocus error signal that is supplied from the signal processing circuit7, and supplies a resulting signal to the comparison circuit 22 b. Theoffset value is used for preventing erroneous detection of a minimumvalue when the focus error signal is influenced by noise or the like.According to an instruction from the microcomputer 13, the comparisoncircuit 22 b compares the offset-value-added output of the adder 18 bwith the output of the value holding circuit 25. If the output value ofthe adder 18 b is smaller than the output value of the value holdingcircuit 25, the comparison circuit 22 b supplies a comparison resultsignal to the value holding circuit 25. In response to the comparisonresult signal, the value holding circuit 25 updates the value that hasbeen held so far to the value of the focus error signal.

The microcomputer 13 judges that the objective lens 3 has made atransition from descent to ascent, at a time point when the comparisonresult signal comes to indicate that the minimum value of the focuserror signal has not been updated. As the objective lens 3 continues todescend, the level of the focus error signal becomes lower than thethreshold level C (23 c) at time point H. The comparison circuit 22 asupplies a comparison detection signal to that effect to themicrocomputer 13. In response to this comparison detection signal, themicrocomputer 13 switches the changeover switch 19 b to the G-side. Thespeed of the objective lens 3 is approximately equal to zero when it isfocused at a position close to the 0th recording layer. Therefore, byperforming a feedback loop focus control using the focus error signal,the objective lens 3 can be pulled into a position where it is focusedon the 0th recording layer.

FIG. 9 shows a case that the lowering voltage and the elevation voltageare well balanced. In this case, the lowering speed of the objectivelens 3 becomes zero when it is focused at a position close to the 0thlayer recording layer, and the objective lens 3 is pulled into aposition where it is focused on the 0th recording layer without making atransition to ascent.

FIG. 10 similarly shows a case that the objective lens 3 is focused onthe first recording layer and it is desired to move the focus positionof the objective lens 3 to the 0th layer recording layer (i.e., it isdesired to jump from a focus point corresponding to the upper (first)recording layer to that corresponding to the lower (0th) recordinglayer), and that the deceleration voltage is too high relative to theacceleration voltage and hence, if no proper measure were taken, theobjective lens 3 would start to ascend before reaching a position whereit is focused on the 0th recording layer.

Referring to FIG. 10, the controls from time point A to G are the sameas described above. That is, the lowering voltage value A (17 a) isapplied first and then the lowering voltage value B (17 b) is applied.After the focus position has passed through the interlayer regionbetween the first recording layer and the 0th recording layer, a speedcontrol is performed by using a signal obtained by the differentiationcircuit 12's differentiating the focus error signal that is suppliedfrom the signal processing circuit 7. The objective lens 3 continues todescend until time point G. Because of the speed control, the loweringspeed becomes zero at a time point close to time point G. The objectivelens 3 starts to ascend because the elevation voltage value A (16 a) isapplied to the actuator after passage of time point G (time point J).Although the focus jump from the first-layer focus point to the0th-layer focus point is intended, a return to the first-layer focuspoint is started.

The focus jump would fail if the above operation continued. To avoidsuch a failure, the operation of returning to the first-layer focuspoint is detected by the above-described minimum value detection by themicrocomputer 13. When detecting the returning operation, themicrocomputer 13 switches the changeover switch 19 b to the G-sidewithout waiting for an event that the level of the focus error signalbecomes lower than the threshold level C (23 c). At this time point, thefocus position of the objective lens 3 is a little distant from the 0threcording layer. However, since the speed of the objective lens 3 isapproximately zero, the objective lens 3 is located in such a regionthat a feedback loop focus control using the focus error signal can beperformed successfully and hence the objective lens 3 can be pulled intoa position where it is focused on the 0th recording layer.

As described above, the way a maximum value and a minimum value occur inthe focus error signal as the objective lens 3 goes up and down may beentirely opposite to the above depending on how the outputs of thephotodetector 54 are connected to the error amplifier 55 (see FIG. 5).Where a maximum value and a minimum value occur in the opposite manner,naturally it is proper to think about the operation of the apparatusbearing in mind that a maximum value and a minimum value occur in theopposite manner for the above reason.

Although in this embodiment the number of positions where the voltagethat is applied to the actuator is controlled by using the focus errorsignal is three, a finer control may be performed by employing morepositions.

In this embodiment, the controls during a focus jump are performed bythe microcomputer 13. FIG. 13 is a PAD diagram showing an algorithm ofthose controls. Based on this algorithm, the microcomputer 13 cancontrol a focus jump in a stable manner.

As described above, this embodiment can provide an optical discapparatus that can perform a focus jump in a stable manner irrespectiveof influence of surface vibration, a variation in interlayer distance,noise that is added to the focus error signal, a variation in thesensitivity of the actuator for driving the objective lens 3,disturbance that is introduced during a focus jump, and other factors bydetecting the movement speed of the objective lens 3 during a focus jumpand variably controlling the deceleration voltage so that the degree ofdeceleration is made constant, and that can perform a focus jump in areliable manner by detecting an event that the objective lens 3 startsto move in the direction opposite to the direction of an intended focusjump by monitoring the focus error signal during the focus jump andthereby preventing the objective lens 3 from returning to a recordinglayer from which the focus jump was started.

FIG. 14 is a block diagram showing an optical disc apparatus and a focusjump method according to another embodiment of the invention. In FIG.14, reference numeral 16 denotes an elevation voltage value; 17, alowering voltage value; 26, a deviation-from-layer preventing elevationvoltage; 27, a deviation-from-layer preventing lowering voltage; 28 a, athreshold level A; 28 b, a threshold level B; 28 c, a threshold level C;28 d, a threshold level D; and 29, a laser power control circuit. Thecircuits in FIG. 14 having the corresponding circuits in FIG. 1 aregiven the same symbols as the latter.

As shown in FIG. 14, a data-recordable disc 1 that has been set on theturn table 2 b is fixed to the turn table 2 b by the clamper 2 a. Thedisc 1 is rotated as the spindle motor 6 is rotated. To read outinformation on the disc 1, the microcomputer 13 controls the laser powercontrol circuit 29 and thereby causes a laser to emit light. Further,the microcomputer 13 controls the laser power control circuit 29 inaccordance with whether to record or reproduce data on or from the disc1.

For example, in the case of a DVD-RAM that is a data-recordable,phase-change disc, information is recorded, erased, and reproduced byutilizing a phase change phenomenon that an alloy film as a recordingfilm is rendered in a crystal state or an amorphous state by applyinglaser light to it and controlling heat generated there.

In recording, high-power laser light is applied to the alloy film, as aresult of which parts of the alloy film are heated to a temperaturehigher than the melting point and are melted. If the melted portions arecooled quickly, they become amorphous. Such an amorphous state is adata-recorded state. On the other hand, in erasure, laser light whosepower is lower than in recording is applied to recorded portions, thatis, amorphous portions, which are thereby heated to a temperature higherthan the crystallization temperature. Those portions are cooled andthereby crystallized. The change from the amorphous state to the crystalstate means data erasure. In general, the crystallization temperature ofan alloy film is lower than its melting temperature. Therefore,high-power laser light whose power is lower than the power of high-powerlaser light that is used in recording can be used in erasure.Information can be recorded and erased by controlling the laser power inthis manner.

In reproduction, low-power laser light whose power is approximately{fraction (1/20)} to {fraction (1/30)} of the power of high-power laserlight used in recording is applied to the alloy film. Data is reproducedby utilizing the fact that crystallized portions and amorphous portionsare different in light reflectance. That is, in an apparatus capable ofrecording and reproducing data on and from a recordable disc, data canbe recorded on, erased from, and reproduced from a disc by using asingle kind of laser light in such a manner that the power of laserlight applied to the disc is controlled in three levels that correspondto recording, erasure, and reproduction, respectively.

In phase-change recordable discs of the above kind, it is a commonprocedure to record and reproduce data by controlling the laser power.

When instructed by the microcomputer 13 to employ a high laser power forrecording or low laser power for reproduction, the laser power controlcircuit 29 supplies an emission control signal to the semiconductorlaser 52 that is incorporated in the pickup 4.

The configuration and the operation of the semiconductor laser 52 andthe optical system of the optical pickup 4 and those, for focus errorsignal detection, of the signal processing circuit 7 are the same asdescribed previously with reference to FIGS. 5 and 6.

A focus error signal generated by the error amplifier 55 (see FIG. 5) issupplied to the focus control circuit 8 (see FIG. 14), which generates,by using a delay compensator, a lead compensator, etc., and outputs afocus control signal for an actuator (not shown) for moving theobjective lens 3, to enable a feedback control in the vicinity of thezero-cross point of an S-shaped curve of the focus error signal. Theoutput signal is supplied to the on/off switch 19 d. In a steady state,the on/off switch 19 d is closed according to an instruction from themicrocomputer 13 and thereby supplies the focus control signal to thepickup 4 as a drive signal. The vertical position of the objective lens3 with respect to the disc 1 is controlled according to this focuscontrol signal and a feedback loop focus control is realized, wherebythe disc 1 always stays at a focus position.

On the other hand, a tracking error signal (TE signal) generated by thesignal processing circuit 7 is supplied to the tracking error controlcircuit 9, which generates, by using a delay compensator, a leadcompensator, etc., to enable a feedback control, a drive signal formoving the objective lens 3 in the horizontal direction (hereinafterreferred to as “tracking direction”) with respect to the disc 1. Thisdrive signal is supplied to the pickup 4. The position of the objectivelens 3 is controlled in the tracking direction according to the drivesignal that is supplied to the inside of the pickup 4, and a feedbackloop tracking control is thereby realized. The beam spot is alwayslocated on pits that are formed in a recording layer of the disc 1. Thedrive signal that is output from the tracking control circuit 9 is alsosupplied to the sled control circuit 10, which generates, by using adelay compensator, a lead compensator, etc., to enable a feedbackcontrol, a drive signal for controlling the sled motor 5 in accordancewith the deviation of the objective lens 3 in the tracking direction.The sled motor 5 is driven according to the drive signal received,whereby the pickup 4 itself is moved.

Further, the signal processing circuit 7 supplies the spindle controlcircuit 11 with rotation cycle information that is read from the disc 1.The spindle control circuit 11 generates by using a delay compensator, alead compensator, etc., a signal for driving the spindle motor 6 basedon the received rotation cycle information, and supplies it to thespindle motor 6. At this time, according to an instruction from themicroprocessor 13, the laser power control circuit 29 controls thesemiconductor laser 52 so that it emits low-power laser light duringdata reproduction and high-power laser light for data recording orerasure during data recording.

In a steady state, recording or reproduction is performed while thefocus, tracking, spindle, and sled (the base of the pickup 4) controlsare performed in the above-described manners with the objective lens 3focused on a recording layer.

Where the disc 1 is a recordable disc of the above kind (e.g., aDVD-RAM) having two recording layers on one side, there may occur a casethat it is necessary to switch from a focus point corresponding to arecording layer on which recording is being performed to another focuspoint corresponding to the other recording layer. Referring to FIG. 15,a description will be made below of an exemplary case that the objectivelens 3 is located at such a position as to be focused on the 0threcording layer and data is being recorded on the 0th recording layer,and that it is desired to move the focus position of the objective lens3 to the first recording layer because data should be recorded on aportion of the first recording layer next (i.e., it is desired to jumpfrom a focus point corresponding to the lower (0th) recording layer onwhich data is being recorded to a focus point corresponding to the upper(first) recording layer).

FIG. 15 shows a displacement variation of the objective lens 3, a focuserror signal, a signal (hereinafter referred to as “differentiationsignal”) obtained by differentiating the focus error signal, a focusdrive signal for driving the objective lens 3, control signals for therespective switches, and a laser power control signal. The vertical axisrepresents the magnitudes of the displacement and the signals, and thehorizontal axis represents time.

First, in a steady state, a focus control signal that is output from thefocus control circuit 8 being in a state that a focus pointcorresponding to the 0th recording layer is established is supplied tothe on/off switch 19 d. Since the on/off switch 19 d is in this steadystate, the focus control signal is supplied to the previous valueholding circuit 15 as it is. The previous value holding circuit 15continues to hold a value that has been held so far and supplies it tothe LPF 14 until the value of the focus control signal changes. The LPF14 has such a frequency characteristic as to reject high-frequencycomponents (noise components) of the signal for driving the objectivelens 3 in the focusing direction and not to reject low-frequencycomponents such as a surface vibration component that is caused byrotation of the disc 1 having a warp or the like. As such,the LPF 14rejects mainly noise components from the drive signal and supplies aresulting signal to the adder 18. In a steady state, the components tothe LPF 14 always operate. Since the feedback loop is now closed, thecontrol of the focus system follows surface vibration and the focuscontrol signal also waves according to the surface vibration.

To cause a focus jump to a focus point corresponding to the firstrecording layer while recording data on the 0th recording layer, themicrocomputer 13 sets initial values of a constant elevation voltagevalue 16 that is an acceleration voltage value necessary for theinterlayer movement, a constant lowering voltage value 17 that is adeceleration voltage value necessary to decelerate and stop, afteracceleration, the objective lens 3 at a position corresponding to afirst-layer focus point, a deviation-from-layer preventing loweringvoltage 27 that is a deceleration voltage value necessary to prevent thefocus position of the objective lens 3 from going past and deviatingfrom the target first recording layer after the focus jump, adeviation-from-layer preventing elevation voltage 26 that is anacceleration voltage value necessary to prevent the objective lens 3from returning to the 0th recording layer from which the focus jumpstarts, a threshold level A (28 a), a threshold level B (28 b), athreshold value C (28 c), a threshold value D (28 d), and a gain factor20. After setting those initial values and before starting the focusjump, the microcomputer 13 controls the laser power control circuit 29so that the semiconductor laser 52, which has so far emitted high-powerlaser light for recording, emits low-power laser light for reproduction.Changing the power of the semiconductor laser 52 to the low power forreproduction prevents an event that recorded data of another recordinglayer or an adjacent track is erased or rewritten erroneously during thefocus jump. After the power of the semiconductor laser 52 has beenchanged to the low power for reproduction, the focus jump is performedin the following manner.

First, the microcomputer 13 switches the changeover switches 19 b and 19c to the H-side and the B-side, respectively, and opens the on/offswitch 19 d. As a result of the switching of the changeover switch 19 band the on/off switch 19 d, the feedback loop by which the objectivelens 3 has been controlled so far is made an open loop and the feedbackcontrol is stopped.

Then, the microcomputer 13 issues, to the changeover switch 19 a, aninstruction to switch to the E-side, whereby the elevation voltage value16 is supplied to the adder 18. The adder 18 adds the elevation voltagevalue 16 to a signal that is free of high-frequency noise components(rejected by the LPF 14), and supplies a resulting addition signal tothe changeover switch 19 c. Since the changeover switch 19 c is switchedto the B-side, the addition signal is supplied from the changeoverswitch 19 c to the changeover switch 19 b as it is. Since the changeoverswitch 19 b is switched to the H-side, the addition signal is suppliedto the pickup 4 via the changeover switch 19 b. Since the elevationvoltage value 16 is applied to the actuator, the objective lens 3 startsto go up.

Referring to FIG. 15, the operation will be described below in detailfor each of sections that are defined by time points A-G.

When the objective lens 3 goes up after the focus jump was started attime point A, the focus error signal rises gradually from a level closeto the middle level until time point B. In this section from time pointA to B, the differentiation signal of the focus error signal risesgradually from a level close to the middle level, reaches a maximumvalue, then gradually decreases, and finally returns to the middle level(zero) at time point B when the focus error signal has a maximum value.As the objective lens 3 goes up further, at time point C the focusposition enters the interlayer region between the 0th recording layerand the first recording layer. The focus error signal decreasesgradually from the maximum value and reaches the middle level (zero). Inthe section from time point B to C, the differentiation signal decreasesfrom the middle level (zero), reaches a minimum value, then increasesgradually, and finally reaches the middle level (zero) again. In thesection from time point C to D that corresponds to the interlayerregion, both of the focus error signal and the differentiation signalare at the middle level (zero). As the objective lens 3 goes up further,the focus position enters the first layer region and hence the focuserror signal falls gradually from a level close to the middle leveluntil time point E. In the section from time point D to E, thedifference signal falls gradually from a level close to the middlelevel, reaches a minimum value, then gradually increases, and finallyreaches the middle level (zero) at time point E when the focus errorsignal has a minimum value. As the objective lens 3 goes up further, afirst-layer focus point is established at time point G. The focus errorsignal gradually increases from the minimum value and reaches the middlelevel (zero). In the section from time point E to G, the differentiationsignal increases from the middle level (zero), reaches a maximum value,then decreases gradually, and finally reaches the middle level (zero).At time point G when the first-layer focus point is established, thefocus error signal and the differentiation signal reach the respectivemiddle levels (zero).

By using the differentiation signal, more specifically, by detecting atime point (zero-cross point) when the differentiation signal crossesthe middle level (zero), a position of the objective lens 3corresponding to time point B can be detected easily and reliably.Although time point B can also be detected by monitoring the level ofthe focus error signal, the detection is not reliable because theamplitude of the focus error signal varies depending on the disc, forexample, and hence is not uniform.

Therefore, the differentiation signal that is output from thedifferentiation circuit 12 is supplied to the microcomputer 13. Themicrocomputer 13 detects that the objective lens 3 has passed a positioncorresponding to time point B by detecting a time point (zero-crosspoint) when the differentiation signal received reaches the middle level(zero).

When detecting the passage of time point B first time, next themicrocomputer 13 detects a zero-cross point (time point C) of the focuserror signal. As the objective lens 3 goes up further, it passes the endposition, corresponding to time point D, of the interlayer regionbetween the 0th recording layer and the first recording layer. Whendetecting time point D by monitoring the level of the focus errorsignal, to apply a voltage value for decelerating the ascendingobjective lens 3 to the actuator, the microcomputer 13 issues, to thechangeover switch 19 c, an instruction to switch to the A-side. Asdescribed above, the focus error signal that is output from the signalprocessing circuit 7 is supplied to the differentiation circuit 12,which generates a differentiation signal and supplies it to themultiplier 21. The multiplier 21 multiplies the received differentiationsignal by the gain factor 20 and supplies a multiplication result to thechangeover switch 19 c. Since at this time the changeover switches 19 band 19 c are switched to the H-side and the A-side, respectively, themultiplication signal obtained by multiplying the differentiation signalby the gain factor 20 is supplied to the actuator as a decelerationvoltage via the changeover switches 19 b and 19 c.

In the section from time point D to E, the focus error signal reflectsthe displacement of the objective lens 3 (it decreases monotonously).Since in general speed is obtained by differentiating displacement withrespect to time, the signal obtained by differentiating the focus errorsignal represents the movement speed of the objective lens 3. Forexample, if a high elevation voltage has been applied to the actuatorand hence an elevation speed of the objective lens 3 when switching ismade to the deceleration voltage is high, the focus error signal fallssteeply from time point D to E. Therefore, a signal obtained bydifferentiating such a focus error signal has a large value, that is,the deceleration voltage has a large value, which means that the forceof decreasing the elevation speed of the objective lens 3 is strong.Conversely, if a low elevation voltage has been applied to the actuatorand hence an elevation speed of the objective lens 3 when switching ismade to the deceleration voltage is low, the focus error signal fallsgently from time point D to E. Therefore, a signal obtained bydifferentiating such a focus error signal has a small value, that is,the deceleration voltage has a small value, which means that the forceof decreasing the elevation speed of the objective lens 3 is weak.

A deceleration voltage value corresponding to an elevation speed of theobjective lens 3 can be obtained and the elevation speed of theobjective lens 3 can be decreased by using a signal obtained bydifferentiating the focus error signal from time D to E in theabove-described manner. The gain factor 20 is used for adjusting theamplitude of the deceleration voltage that is obtained bydifferentiating the focus error signal.

The objective lens 3 continues to go up owing to the acceleration thatwas given by the elevation voltage 16 even after the application of thedeceleration voltage to the actuator.

After the application of the deceleration voltage to the actuator, themicrocomputer 13 judges that time point E has been passed by detecting atime point (zero-cross point) when the differentiation signal that issupplied from the differentiation circuit 12 reaches the middle level(zero) again. When detecting the passage of time point E, to stop stablythe objective lens 3 that is about to make a transition from ascent todescent and to establish a first-layer focus point (time point G in FIG.15), the microcomputer 13 issues, to the changeover switches 19 a and 19c, instructions to switch to the F-side and the B-side, respectively.Switched to the F-side, the changeover switch 19 a outputs the loweringvoltage value 17. The adder 18 adds together the lowering voltage value17 and a signal that is free of high-frequency noise components(rejected by the LPF 14), and supplies a resulting addition signal tothe changeover switch 19 c. Since the changeover switch 19 c is switchedto the B-side, the addition signal is supplied from the changeoverswitch 19 c to the changeover switch 19 b as it is. Since the changeoverswitch 19 b is switched to the H-side, the addition signal is supplied,via the changeover switch 19 b, to the pickup 4, where it is applied tothe actuator for driving the objective lens 3. Because of theapplication of the lowering voltage value 17 to the actuator, theelevation speed of the objective lens 3 is decreased and the objectivelens 3 stops ascending.

After the objective lens 3 has passed the position corresponding to timepoint E, the microcomputer 13 issues, to the changeover switch 19 e, toswitch to the N-side, whereby the threshold level D (28 d) is suppliedto the comparison circuit 22. The comparison circuit 22 compares thefocus error signal that is supplied from the signal processing circuit 7with the threshold level D (28 d), and supplies a comparison result tothe microcomputer 13. As the objective lens 3 continues to ascend, thelevel of the focus error signal exceeds the threshold level D (28 d) attime point F. The comparison circuit 22 supplies a comparison detectionsignal to that effect to the microcomputer 13. In response to thiscomparison detection signal, the microcomputer 13 switches thechangeover switch 19 b to the G-side. The speed of the objective lens 3is approximately equal to zero when it is focused at a position close tothe first recording layer. Therefore, a feedback loop focus controlusing the focus error signal is started, whereby the objective lens 3can be pulled into a position where it is focused on the first recordinglayer.

FIG. 15 shows a case that the elevation voltage and the lowering voltageare well balanced. In this case,the elevation speed of the objectivelens 3 becomes zero when it is focused at a position close to the firstlayer recording layer, and the objective lens 3 is pulled into aposition where it is focused on the first recording layer without makinga transition to descent.

After the objective lens 3 is pulled into the position where it isfocused on the first recording layer, the current position of theobjective lens 3 is detected based on an ID or the like and theobjective lens 3 is moved in the direction (tracking direction) parallelwith the disc 1 to a target position where recording on the firstrecording layer should be started. After the objective lens 3 is movedto the target position, the microcomputer 13 controls the laser powercontrol circuit 29 so that the semiconductor laser 52 (see FIG. 5),which has so far emitted low-power laser light for reproduction, emitshigh-power laser light for recording. This makes it possible to recorddata on an intended portion of the first recording layer.

FIG. 16 similarly shows a case that the objective lens 3 is focused onthe 0th recording layer and it is desired to move the focus position ofthe objective lens 3 to the first layer recording layer (i.e., it isdesired to jump from a focus point corresponding to the lower (0th)recording layer to that corresponding to the upper (first) recordinglayer), and that the acceleration voltage (elevation voltage 16) is toohigh relative to the deceleration voltage (lowering voltage 17) andhence, if no proper measure were taken, the objective lens 3 wouldcontinue to ascend even after reaching a position where it is focused onthe upper (first) recording layer (i.e., an upper-layer focus pointwould be passed) and reach a position where a feedback loop focuscontrol for the upper recording layer cannot be performedsatisfactorily.

Referring to FIG. 16, the controls on the objective lens 3 from timepoint A to F are the same as described above. That is, the elevationvoltage value 16 is applied first. After the focus position has passedthrough the interlayer region between the 0th recording layer and thefirst recording layer, a speed control is performed by using a signalobtained by the differentiation circuit 12's differentiating the focuserror signal that is supplied from the signal processing circuit 7.

In the above description that was made with reference to FIG. 15, it wasassumed that the speed control makes the elevation speed sufficientlylow in the vicinity of time point E, after time point E the applicationof the lowering voltage value 17 further decreases the elevation speed,and the elevation speed becomes zero in the vicinity of time point G. Incontrast, in the case of FIG. 16, the elevation speed after time point Eis so high that the application of the lowering voltage value 17 cannotbrake the objective lens 3 sufficiently and its elevation speed is notdecreased much. The feedback loop focus control using the focus errorsignal that is started at time point F cannot decrease the elevationspeed to zero even at time point G when the objective lens 3 is focusedon the upper (first) recording layer. The objective lens 3 goes past anddeviates from the position corresponding to time point G where it isfocused on the upper recording layer; pulling into focus cannot beattained.

In view of the above, after detecting the passage of time point E, themicrocomputer 13 issues, to the changeover switch 19 e, an instructionto switch to the N-side, whereby the threshold level D (28 d) issupplied to the comparison circuit 22. The comparison circuit 22compares the focus error signal that is supplied from the signalprocessing circuit 7 with the threshold level D (28 d). As the objectivelens 3 continues to ascend, the level of the focus error signal exceedsthe threshold level D (28 d) at time point F. The comparison circuit 22supplies a comparison detection signal to that effect to themicrocomputer 13. In response to this comparison detection signal, themicrocomputer 13 switches the changeover switch 19 b to the G-side,whereby a feedback loop focus control using the focus error signal isstarted.

After the passage of time point F, the microcomputer 13 issues, to thechangeover switch 19 e, an instruction to switch to the K-side, wherebythe threshold level A (28 a) is supplied to the comparison circuit 22.The comparison circuit 22 compares the focus error signal that issupplied from the signal processing circuit 7 with the threshold level A(28 a) and supplies a comparison result to the microcomputer 13.

Although the feedback loop focus control is performed, the elevationspeed is not decreased. As the objective lens 3 goes up further, timepoint G is passed and the level of the focus error signal exceeds thethreshold level A (28 a) at time point H. The comparison circuit 22supplies a comparison detection signal to that effect to themicrocomputer 13. detecting the passage of time point H based on the4comparison detection signal, the microcomputer 13 switches thechangeover switch 19 b to the H-side and opens the on/off switch 19 d.Because of the switching of the changeover switch 19 b and the on/offswitch 19 d, the feedback loop that has been used so far for controllingthe objective lens 3 is again made an open loop.

At this time, the microcomputer 13 issues, to the changeover switches 19a and 19 c, instructions to switch to the D-side and B-side,respectively. Because of the switching of the changeover switch 19 a tothe D-side, the changeover switch 19 a outputs the deviation-from-layerpreventing lowering voltage 27. The adder 18 adds together a signal thatis free of high-frequency components (rejected by the LPF 14) and thedeviation-from-layer preventing lowering voltage 27, and supplies aresulting addition signal to the changeover switch 19 c. Since thechangeover switch 19 c is switched to the B-side, the addition signal issupplied from the changeover switch 19 c to the changeover switch 19 bas it is. Since the changeover switch 19 b is switched to the H-side,the addition signal is supplied, via the changeover switch 19 b, to thepickup 4, where it is applied to the actuator for driving the objectivelens 3. As a result, because of the application of the lowering voltage27 to the actuator, the elevation speed of the ascending objective lens3 is decreased to zero and the objective lens 3 starts to go down.

Since the objective lens 3 goes down, its focus position can beprevented from deviating from the upper recording layer. As theobjective lens 3 goes down, the microcomputer 13 issues, to thechangeover switch 19 e, an instruction to switch to the M-side, wherebythe threshold level C (28 c) is supplied to the comparison circuit 22.The comparison circuit 22 compares the focus error signal that issupplied from the signal processing circuit 7 with the threshold level C(28 c) and supplies a comparison result to the microcomputer 13. As theobjective lens 3 goes down further, the level of the focus error signalbecomes lower than the threshold level C (28 c) at time point I. Thecomparison circuit 22 supplies a comparison detection signal to thateffect to the microcomputer 13. In response to the comparison detectionsignal, the microcomputer 13 switches the changeover switch 19 b to theG-side, whereby a feedback loop focus control using the focus errorsignal is started. At this time, the focus position of the objectivelens 3 is close to the first recording layer and is located in such aregion that a feedback loop focus control using the focus error signalcan be performed successfully. Further, the lowering speed of theobjective lens 3 is so low that a feedback loop focus control can beperformed successfully. Therefore, the objective lens 3 can be pulledinto a position where it is focused on the first recording layer.

After the objective lens 3 is pulled into the position where it isfocused on the first recording layer, the current position of theobjective lens 3 is detected based on an ID or the like and theobjective lens 3 is moved in the direction (tracking direction) parallelwith the disc 1 to a target position where recording on the firstrecording layer should be started. After the objective lens 3 is movedto the target position, the microcomputer 13 controls the laser powercontrol circuit 29 so that the semiconductor laser 52 (see FIG. 5),which has so far emitted low-power laser light for reproduction, emitshigh-power laser light for recording. This makes it possible to recorddata on an intended portion of the first recording layer.

FIG. 17 shows a case that the objective lens 3 is located at such aposition as to be focused on the first recording layer and data is beingrecorded on the first recording layer, and that it is desired to movethe focus position of the objective lens 3 to the 0th recording layerbecause data should be recorded on a portion of the 0th recording layernext (i.e., it is desired to jump from a focus point corresponding tothe upper (first) recording layer on which data is being recorded to afocus point corresponding to the lower (0th) recording layer). FIG. 17shows a displacement variation of the objective lens 3, a focus errorsignal, a signal (hereinafter referred to as “differentiation signal”)obtained by differentiating the focus error signal, a focus drive signalfor driving the objective lens 3, control signals for the srespectiveswitches, and a laser power control signal. The vertical axis representsthe magnitudes of the displacement and the signals, and the horizontalaxis represents time.

First, it is assumed that the apparatus is in a steady state in which afeedback loop focus control is being performed in a state that a focuspoint corresponding to the first recording layer is established.

To cause a focus jump to a focus point corresponding to the 0threcording layer while recording data on the first recording layer, themicrocomputer 13 sets initial values of a constant lowering voltagevalue 17 that is an acceleration voltage value necessary for theinterlayer movement, a constant elevation voltage value 16 that is adeceleration voltage value necessary to decelerate and stop, afteracceleration, the objective lens 3 at a position corresponding to a0th-layer focus point, a deviation-from-layer preventing elevationvoltage 26 that is a deceleration voltage value necessary to prevent thefocus position of the objective lens 3 from going past and deviatingfrom the target 0th recording layer after the focus jump, adeviation-from-layer preventing lowering voltage 27 that is anacceleration voltage value necessary to prevent the objective lens 3from returning to the first recording layer from which the focus jumpstarted, a threshold level A (28 a), a threshold level B (28 b), athreshold value C (28 c), a threshold value D (28 d), and a gain factor20.

After setting those initial values and before starting the focus jump,the microcomputer 13 controls the laser power control circuit 29 so thatthe semiconductor laser 52, which has so far emitted high-power laserlight for recording, emits low-power laser light for reproduction.Changing the power of the semiconductor laser 52 to the low power forreproduction prevents an event that recorded data of another recordinglayer or an adjacent track is erased or rewritten erroneously during thefocus jump.

After the power of the semiconductor laser 52 has been changed to thelow power for reproduction, the focus jump is performed in the followingmanner. First, the microcomputer 13 switches the changeover switches 19b and 19 c to the H-side and the B-side, respectively, and opens theon/off switch 19 d. As a result of the switching of the changeoverswitch 19 b and the on/off switch 19 d, the feedback loop by which theobjective lens 3 has been controlled so far is made an open loop and thefocus control is stopped.

Then, the microcomputer 13 issues, to the changeover switch 19 a, aninstruction to switch to the F-side, whereby the lowering voltage value17 is supplied to the adder 18. The adder 18 adds the lowering voltagevalue 17 to a signal that is free of high-frequency noise components(rejected by the LPF 14), and supplies a resulting addition signal tothe changeover switch 19 c. Since the changeover switch 19 c is switchto the B-side, the addition signal is supplied from the changeoverswitch 19 c to the changeover switch 19 b as it is. Since the changeoverswitch 19 b is switched to the H-side, the addition signal is suppliedto the pickup 4 via the changeover switch 19 b. Since the loweringvoltage value 17 is applied to the actuator, the objective lens 3 startsto go down.

Referring to FIG. 17, the operation will be described below in detailfor each of sections that are defined by time points A-G.

When the objective lens 3 goes down after the focus jump was started attime point A, the focus error signal falls gradually from a level closeto the middle level until time point B. In this section from time pointA to B, a signal (a focus error differentiation signal (hereinafterreferred to merely as “differentiation signal”)) obtained bydifferentiating the focus error signal falls gradually from a levelclose to the middle level, reaches a minimum value, then graduallyincreases, and finally returns to the middle level (zero) at time pointB when the focus error signal has a minimum value. As the objective lens3 goes down further, at time point C the focus position enters theinterlayer region between the first recording layer and the 0threcording layer. The focus error signal increases gradually from theminimum value and reaches the middle level (zero). In the section fromtime point B to C, the differentiation signal increases from the middlelevel (zero), reaches a maximum value, then decreases gradually, andfinally reaches the middle level (zero) again. In the section from timepoint C to D that corresponds to the interlayer region, both of thefocus error signal and the differentiation signal are at the middlelevel (zero). As the objective lens 3 goes down further, the focusposition enters the 0th layer region and hence the focus error signalrises gradually from a level close to the middle level until time pointE. In the section from time point D to E, the difference signal risesgradually from a level close to the middle level, reaches a maximumvalue, then gradually decreases, and finally reaches the middle level(zero) at time point E when the focus error signal has a maximum value.As the objective lens 3 goes down further, a 0th-layer focus point isestablished at time point G. The focus error signal gradually decreasesfrom the maximum value and reaches the middle level (zero). In thesection from time point E to G, the differentiation signal decreasesfrom the middle level (zero), reaches a minimum value, then increasesgradually, and finally reaches the middle level (zero). At time point Gwhen the 0th-layer focus point is established, the focus error signaland the differentiation signal reach the respective middle levels(zero).

By using the differentiation signal, more specifically, by detecting atime point (zero-cross point) when the differentiation signal crossesthe middle level (zero), a position of the objective lens 3corresponding to time point B can be detected easily and reliably.Although time point B can also be detected by monitoring the level ofthe focus error signal, the detection is not reliable because theamplitude of the focus error signal varies depending on the disc, forexample, and hence is not uniform.

Therefore, the differentiation signal that is output from thedifferentiation circuit 12 is supplied to the microcomputer 13. Themicrocomputer 13 detects that the objective lens 3 has passed a positioncorresponding to time point B by detecting a time point (zero-crosspoint) when the differentiation signal received reaches the middle level(zero). When detecting the passage of time point B first time, next themicrocomputer 13 detects a zero-cross point (time point C) of the focuserror signal. As the objective lens 3 goes down further, it passes theend position, corresponding to time point D, of the interlayer regionbetween the first recording layer and the 0th recording layer. Whendetecting time point D by monitoring the level of the focus errorsignal, to apply a voltage value for decelerating the descendingobjective lens 3 to the actuator, the microcomputer 13 issues, to thechangeover switch 19 c, an instruction to switch to the A-side.

On the other hand, the focus error signal that is output from the signalprocessing circuit 7 is supplied to the differentiation circuit 12,which generates a differentiation signal and supplies it to themultiplier 21. The multiplier 21 multiplies the received differentiationsignal by the gain factor 20 and supplies a multiplication result to thechangeover switch 19 c. Since at this time the changeover switch 19 c isswitched to the A-side, the multiplication signal is supplied to thechangeover switch 19 b via the changeover switch 19 c as it is. Sincethe change over switch 19 b is switched to the H-side, themultiplication signal that was obtained by multiplying thedifferentiation signal by the gain factor 20 is supplied to the actuatoras a deceleration voltage via the changeover switch 19 b.

In the section from time point D to E, the focus error signal reflectsthe displacement of the objective lens 3 (it increases monotonously). Asdescribed above, the signal obtained by differentiating the focus errorsignal represents the movement speed of the objective lens 3. Forexample, if a high lowering voltage has been applied to the actuator andhence a lowering speed of the objective lens 3 when switching is made tothe deceleration voltage is high, the focus error signal rises steeplyfrom time point D to E. Therefore, a signal obtained by differentiatingsuch a focus error signal has a large value, that is, the decelerationvoltage has a large value, which means that the force of decreasing thelowering speed of the objective lens 3 is strong. Conversely, if a lowlowering voltage has been applied to the actuator and hence a loweringspeed of the objective lens 3 when switching is made to the decelerationvoltage is low, the focus error signal rises gently from time point D toE. Therefore, a signal obtained by differentiating such a focus errorsignal has a small value, that is, the deceleration voltage has a smallvalue, which means that the force of decreasing the lowering speed ofthe objective lens 3 is weak.

A deceleration voltage value corresponding to a lowering speed of theobjective lens 3 can be obtained and the lowering speed of the objectivelens 3 can be decreased by using a signal obtained by differentiatingthe focus error signal from time D to E in the above-described manner.The gain factor 20 is used for adjusting the amplitude of thedeceleration voltage (differentiation voltage) that is obtained bydifferentiating the focus error signal.

The objective lens 3 continues to go down owing to the acceleration thatwas given by the lowering voltage 17 even after the application of thedeceleration voltage to the actuator. After the application of thedeceleration voltage to the actuator, the microcomputer 13 judges thattime point E has been passed by detecting a time point (zero-crosspoint) when the differentiation signal that is supplied from thedifferentiation circuit 12 reaches the middle level (zero) again. Whendetecting the passage of time point E, to stop stably the objective lens3 that is about to make a transition from descent to ascent and toestablish a 0th-layer focus point (time point G in FIG. 17), themicrocomputer 13 issues, to the changeover switches 19 a and 19 c,instructions to switch to the E-side and the B-side, respectively.Switched to the E-side, the changeover switch 19 a outputs the elevationvoltage value 16.

The adder 18 adds together the elevation voltage value 16 and a signalthat is free of high-frequency noise components (rejected by the LPF14), and supplies a resulting addition signal to the changeover switch19 c. Since the changeover switch 19 c is switched to the B-side, theaddition signal is supplied from the changeover switch 19 c to thechangeover switch 19 b as it is. Since the changeover switch 19 b isswitched to the H-side, the addition signal is supplied, via thechangeover switch 19 b, to the pickup 4, where it is applied to theactuator for driving the objective lens 3. Because of the application ofthe elevation voltage value 16 to the actuator, the lowering speed ofthe objective lens 3 is decreased and the objective lens 3 stopsdescending.

After detecting the passage of time point E, the microcomputer 13issues, to the changeover switch 19 e, to switch to the M-side, wherebythe threshold level C (28 c) is supplied to the comparison circuit 22.The comparison circuit 22 compares the focus error signal that issupplied from the signal processing circuit 7 with the threshold level C(28 c), and supplies a comparison result to the microcomputer 13. As theobjective lens 3 continues to descend, the level of the focus errorsignal becomes lower than the threshold level C (28 c) at time point F.The comparison circuit 22 supplies a comparison detection signal to thateffect to the microcomputer 13. In response to this comparison detectionsignal, the microcomputer 13 switches the changeover switch 19 b to theG-side. The speed of the objective lens 3 is equal to zero when it isfocused at a position close to the 0th recording layer. Therefore, afeedback loop focus control using the focus error signal is started,whereby the objective lens 3 can be pulled into a position where it isfocused on the 0th recording layer.

FIG. 17 shows a case that the elevation voltage and the lowering voltageare well balanced. In this case, the elevation speed of the objectivelens 3 becomes zero when it is focused at a position close to the firstlayer recording layer, and the objective lens 3 is pulled into aposition where it is focused on the first recording layer without makinga transition to ascent.

After the objective lens 3 is pulled into the position where it isfocused on the 0th recording layer, the current position of theobjective lens 3 is detected based on an ID or the like and theobjective lens 3 is moved in the direction (tracking direction) parallelwith the disc 1 to a target position where recording on the 0threcording layer should be started. After the objective lens 3 is movedto the target position, the microcomputer 13 controls the laser powercontrol circuit 29 so that the semiconductor laser 52 (see FIG. 5),which has so far emitted low-power laser light for reproduction, emitshigh-power laser light for recording. This makes it possible to recorddata on an intended portion of the first recording layer.

FIG. 18 similarly shows a case that the objective lens 3 is focused onthe first recording layer and it is desired to move the focus positionof the objective lens 3 to the 0th layer recording layer (i.e., it isdesired to jump from a focus point corresponding to the upper (first)recording layer to that corresponding to the lower (0th) recordinglayer), and that the acceleration voltage (lowering voltage 17) is toohigh relative to the deceleration voltage (elevation voltage 16) andhence, if no proper measure were taken, the objective lens 3 wouldcontinue to descend even after reaching a position where it is focusedon the lower (0th) recording layer (i.e., a lower-layer focus pointwould be passed) and reach a position where a feedback loop focuscontrol for the lower recording layer cannot be performedsatisfactorily.

Referring to FIG. 18, the controls on the objective lens 3 from timepoint A to F are the same as described above. That is, the loweringvoltage value 17 is applied first. After the focus position has passedthrough the interlayer region between the first recording layer and the0th recording layer, a speed control is performed by using a signalobtained by the differentiation circuit 12's differentiating the focuserror signal that is supplied from the signal processing circuit 7.

In the above description that was made with reference to FIG. 17, it wasassumed that the speed control makes the lowering speed sufficiently lowin the vicinity of time point E, after time point E the application ofthe elevation voltage value 16 further decreases the lowering speed, andthe lowering speed becomes zero in the vicinity of time point G. Incontrast, in the case of FIG. 18, the lowering speed after time point Eis so high that the application of the elevation voltage value 16 cannotbrake the objective lens 3 sufficiently and its elevation speed is notdecreased much. The feedback loop focus control using the focus errorsignal that is started at time point F cannot decrease the loweringspeed to zero even at time point G when the objective lens 3 is focusedon the lower (0th) recording layer. The objective lens 3 goes past anddeviates from the position corresponding to time point G where it isfocused on the lower recording layer; pulling into focus cannot beattained.

In view of the above, after detecting the passage of time point E, themicrocomputer 13 issues, to the changeover switch 19 e, an instructionto switch to the M-side, whereby the threshold level C (28 c) issupplied to the comparison circuit 22. The comparison circuit 22compares the focus error signal that is supplied from the signalprocessing circuit 7 with the threshold level C (28 c). As the objectivelens 3 continues to descend, the level of the focus error signal becomeslower the threshold level C (28 c) at time point F. The comparisoncircuit 22 supplies a comparison detection signal to that effect to themicrocomputer 13. In response to this comparison detection signal, themicrocomputer 13 switches the changeover switch 19 b to the G-side,whereby a feedback loop focus control using the focus error signal isstarted.

After detecting the passage of time point F, the microcomputer 13issues, to the changeover switch 19 e, an instruction to switch to theL-side, whereby the threshold level B (28 b) is supplied to thecomparison circuit 22. The comparison circuit 22 compares the focuserror signal that is supplied from the signal processing circuit 7 withthe threshold level B (28 b) and supplies a comparison result to themicrocomputer 13. Although the feedback loop focus control is performed,the lowering speed is not decreased. As the objective lens 3 goes downfurther, time point G is passed and the level of the focus error signalbecomes lower than the threshold level B (28 b) at time point H. Thecomparison circuit 22 supplies a comparison detection signal to thateffect to the microcomputer 13. Detecting the passage of time point Hbased on the comparison detection signal, the microcomputer 13 switchesthe changeover switch 19 b to the H-side and opens the on/off switch 19d. Because of the switching of the changeover switch 19 b and the on/offswitch 19 d, the feedback loop that has been used so far for controllingthe objective lens 3 is again made an open loop.

At this time, the microcomputer 13 issues, to the changeover switches 19a and 19 c, instructions to switch to the C-side and B-side,respectively. Because of the switching of the changeover switch 19 a tothe C-side, the changeover switch 19 a outputs the deviation-from-layerpreventing elevation voltage 26. The adder 18 adds together a signalthat is free of high-frequency components (rejected by the LPF 14) andthe deviation-from-layer preventing elevation voltage 26, and supplies aresulting addition signal to the changeover switch 19 c. Since thechangeover switch 19 c is switched to the B-side, the addition signal issupplied from the changeover switch 19 c to the changeover switch 19 bas it is. Since the changeover switch 19 b is switched to the H-side,the addition signal is supplied, via the changeover switch 19 b, to thepickup 4, where it is applied to the actuator for driving the objectivelens 3. As a result, because of the application of the elevation voltage26 to the actuator, the lowering speed of the descending objective lens3 is decreased to zero and the objective lens 3 starts to go up. Sincethe objective lens 3 goes up, its focus position can be prevented fromdeviating from the lower recording layer.

As the objective lens 3 goes up, the microcomputer 13 issues, to thechangeover switch 19 e, an instruction to switch to the N-side, wherebythe threshold level D (28 d) is supplied to the comparison circuit 22.The comparison circuit 22 compares the focus error signal that issupplied from the signal processing circuit 7 with the threshold level D(28 d) and supplies a comparison result to the microcomputer 13.

As the objective lens 3 goes up further, the level of the focus errorsignal exceeds the threshold level D (28 d) at time point I. Thecomparison circuit 22 supplies a comparison detection signal to thateffect to the microcomputer 13. In response to the comparison detectionsignal, the microcomputer 13 switches the changeover switch 19 b to theG-side, whereby a feedback loop focus control using the focus errorsignal is started. At this time, the focus position of the objectivelens 3 is close to the 0th recording layer and is located in such aregion that a feedback loop focus control using the focus error signalcan be performed successfully. Further, the elevation speed of theobjective lens 3 is so low that a feedback loop focus control can beperformed successfully. Therefore, the objective lens 3 can be pulledinto a position where it is focused on the 0th recording layer.

After the objective lens 3 is pulled into the position where it isfocused on the 0th recording layer, the current position of theobjective lens 3 is detected based on an ID or the like and theobjective lens 3 is moved in the direction (tracking direction) parallelwith the disc 1 to a target position where recording on the 0threcording layer should be started. After the objective lens 3 is movedto the target position, the microcomputer 13 controls the laser powercontrol circuit 29 so that the semiconductor laser 52 (see FIG. 5),whichhas so far emitted low-power laser light for reproduction, emitshigh-power laser light for recording. This makes it possible to recorddata on an intended portion of the 0th recording layer.

The way a maximum value and a minimum value occur in the focus errorsignal as the objective lens 3 goes up and down may be entirely oppositeto the above depending on how the outputs of the photodetector 54 areconnected to the error amplifier 55 (see FIG. 5). Where a maximum valueand a minimum value occur in the opposite manner, naturally it is properto think about the operation of the apparatus bearing in mind that amaximum value and a minimum value occur in the opposite manner for theabove reason.

A focus jump during data recording, which has been described above, isnecessary in a case where during continuous data recording there occursdata whose addresses bridge two recording layers. Such a focus jump isstarted in processing an address portion that is not to be recorded onthe disc 1 after writing or a data portion. Data can be recordedcontinuously after the target address is reached.

The above-described individual controls of a focus jump during datarecording are performed by the microcomputer 13. FIG. 19 shows analgorithm of those controls.

Referring to FIG. 19, to start a focus jump during recording, first, atstep 100 physical address information that is stored in the optical disc1 is acquired and the current address of the objective lens 3 ischecked. At step 101, it is judged whether the recording layer on whichdata should be recorded next is the same layer as the current recordinglayer. If it is judged at step 101 that the recording layer on whichdata should be recorded next is the same layer as the current recordinglayer, it is not necessary to perform a focus jump and hence the processis finished. On the other hand, if it is judged that the recording layeron which data should be recorded next is not the current recordinglayer, a focus jump is performed. Before starting a focus jump, thelaser power is changed to the low power for reproduction. The recordingis suspended at this stage (step 102). When the laser power has becomethe lower power for reproduction, focus jump processing (describedabove) is performed at step 103.

After the focus jump processing has been performed, at step 104 focusservo processing is performed so that the focus position of theobjective lens 3 is pulled into the target recording layer. At step 105,deviation-from-layer detection processing (described above) isperformed. If it is judged at step 105 that the feedback loop focuscontrol cannot attain pulling into focus and the focus position of theobjective lens 3 deviates from the target recording layer,deviation-from-layer preventing processing is performed at step 106. Atstep 107, focus servo processing is performed again so that the focusposition of the objective lens 3 is pulled into the target recordinglayer. At step 108, a current position is acquired. At step 109, it ischecked whether the focus jump to the recording layer on which datashould be recorded next has been performed normally. This can be judgedby monitoring a focus error signal or some other servo-related signalduring or after the focus jump or by using the post-jump current addressthat was acquired at step 108.

If the focus jump to the recording layer on which data should berecorded next has succeeded, at step 110 the objective lens 3 is movedin the direction (tracking direction) parallel with the disc 1 to atarget position from which data recording should be restarted. After theobjective lens 3 has been moved to the position from which datarecording should be restarted, at step 111 the laser power is increasedto the high power for recording and data recording is restarted.

If it is judged at step 109 that the focus jump has failed, servorecovery processing is performed at step 112. In general, if a focusjump does not result in pulling into a target recording layer, thesubjects of the focus control and the other controls are out of thosecontrols. Therefore, servo recovery processing such as re-pulling-in ofthe focus control are performed when necessary. After the focus controland other servo controls have recovered, step 100 and the followingsteps are executed again. According to the above algorithm, themicrocomputer 13 can control a focus jump in a stable manner.

As described above, in this embodiment, in performing a focus jumpduring recording on a disc having a plurality of data-recordablerecording layers, the laser power is controlled so as to become too lowto effect recording, whereby erroneous erasure of an already recordedportion can be prevented and a focus jump can be performed even duringrecording. Further, an event that the focus position of the objectivelens 3 is going to deviate from the target recording layer is detectedat the end of a focus jump by detecting that the level of a focus errorsignal exceeds a set threshold level, and the actuator is controlled soas to prevent the focus position of the objective lens 3 from deviatingfrom the target recording layer. This enables a stable focus jump evenduring recording.

As described above, the invention provides advantages that a focus jumpcan be performed in a stable manner by detecting the movement speed ofthe objective lens during a focus jump and variably controlling adeceleration voltage so that the degree of deceleration is madeconstant, and that a focus jump can be performed in a reliable manner bydetecting an event that the objective lens starts to move in thedirection opposite to the direction of an intended focus jump bymonitoring a focus error signal during the focus jump and therebypreventing the objective lens from returning to a recording layer fromwhich the focus jump was started.

Further, as for a focus jump during recording on a disc having aplurality of data-recordable recording layers, the invention providesadvantages that erroneous erasure of an already recorded portion can beprevented and a focus jump can be performed even during recording bycontrolling the laser power so that it becomes too low to effectrecording, and that a focus jump can be performed in a stable andreliable manner even during recording by detecting an event that thefocus position of the objective lens is going to deviate from the targetrecording layer at the end of a focus jump by detecting that the levelof a focus error signal exceeds a set threshold level and controllingthe actuator so as to prevent the focus position of the objective lensfrom deviating from the target recording layer.

1. An optical disc apparatus having a focus jump function for enabling afocus control on each of a plurality of recording layers of a disc onand from which data can be recorded and reproduced, comprising: anobjective lens for focusing laser light on a recording layer of thedisc; focus error signal generating means for generating a focus errorsignal based on reflection light that is obtained through the objectivelens; generating means for generating, based on the focus error signal,a focus control signal for controlling the objective lens; drive voltagegenerating means for outputting a voltage necessary to move theobjective lens; moving means for moving the objective lens in adirection approximately perpendicular to the recording layers of thedisc in accordance with the output voltage of the drive voltagegenerating means; and control means for starting, when a focus jumpbecomes necessary during data recording, the focus jump after switchinglaser light power that is currently made high to enable the datarecording to such a low power that neither data recording nor erasurecan be effected.
 2. An optical disc apparatus having a focus jumpfunction for enabling a focus control on each of a plurality ofrecording layers of a disc on and from which data can be recorded andreproduced, comprising: an objective lens for focusing laser light on arecording layer of the disc; a signal processing circuit for generatinga focus error signal based on reflection light that is obtained throughthe objective lens; a focus control circuit for generating, based on thefocus error signal, a focus control signal for controlling the objectivelens; a drive voltage generating circuit for outputting a drive voltagenecessary to move a focus position of the objective lens betweenrecording layers; an actuator for moving the objective lens in adirection approximately perpendicular to the recording layers of thedisc in accordance with the output voltage of the drive voltagegenerating circuit; and a control circuit for starting, when a focusjump becomes necessary during data recording, the focus jump afterperforming a control of switching laser light power that is currentlymade high to enable the data recording to such a low power that neitherdata recording nor erasure can be effected.
 3. An optical disc apparatushaving a focus jump function for enabling a focus control on each of aplurality of recording layers of a disc on and from which data can berecorded and reproduced, comprising: an objective lens for focusinglaser light on a recording layer of the disc; focus error signalgenerating means for generating a focus error signal based on reflectionlight that is obtained through the objective lens; generating means forgenerating, based on the focus error signal, a focus control signal forcontrolling the objective lens; drive voltage generating means foroutputting a voltage necessary to move the objective lens; moving meansfor moving the objective lens in a direction approximately perpendicularto the recording layers of the disc in accordance with the outputvoltage of the drive voltage generating means; means for controllingpower of a laser that is used for recording and reproducing data on andfrom the disk; and control means for starting, when a focus jump becomesnecessary during data recording, the focus jump after switching laserlight power that is currently made high to enable the data recording tosuch a low power that neither data recording nor erasure can beeffected.
 4. An optical disc apparatus having a focus jump function forenabling a focus control on each of a plurality of recording layers of adisc on and from which data can be recorded and reproduced, comprising;an objective lens for focusing laser light on a recording layer of thedisc; a signal processing circuit for generating a focus error signalbased on reflection light that is obtained through the objective lens; afocus control circuit for generating, based on the focus error signal, afocus control signal for controlling the objective lens; a drive voltagegenerating circuit for outputting a drive voltage necessary to move afocus position of the objective lens between recording layers; anactuator for moving the objective lens in a direction approximatelyperpendicular to the recording layers of the disc in accordance with theoutput voltage of the drive voltage generating circuit; a laser powercontrol circuit for controlling power of a laser that is used forrecording and reproducing data on and from the disk; and a controlcircuit for starting, when a focus jump becomes necessary during datarecording, the focus jump after performing a control of switching laserlight power that is currently made high to enable the data recording tosuch a low power that neither data recording nor erasure can be effectedby controlling the laser power control circuit.
 5. The optical discapparatus according to claim 1 wherein the drive voltage generatingmeans generates: a first voltage value as an acceleration voltage thatcauses the objective lens to approach the disc and a second voltagevalue as a deceleration voltage that causes the objective lens to goaway from the disc in moving the objective lens from a first recordinglayer to a second recording layer that is more distant from theobjective lens than the first recording layer is, and a third voltagevalue as an acceleration voltage that causes the objective lens to goaway from the disc and a fourth voltage value as a deceleration voltagethat causes the objective lens to approach the disc in moving theobjective lens from a third recording layer to a fourth recording layerthat is closer to the objective lens than the third recording layer is.6. A focus jump method of an optical disc apparatus having a focus jumpfunction for enabling a focus control on each of a plurality ofrecording layers of a disc on and from which data can be recorded andreproduced, comprising the steps of: detecting a current position of anobjective lens while recording data on the disc; judging whether aposition where to record data next is located in a recording layer onwhich the objective lens is currently focused; if it is judged that theposition where to record data next is not located in the recording layeron which the objective lens is currently focused and hence a first focusjump is necessary, switching laser power from a high power for datarecording to such a low power that neither data recording nor erasurecan be effected; performing the first focus jump after switching thelaser power to the low power; judging whether a focus position of theobjective lens will deviate from a target recording layer based on alevel of a focus error signal that is obtained when the focus positionof the objective lens reaches the target recording layer as a result ofthe first focus jump; if it is judged that the focus position of theobjective lens will deviate from the target recording layer, performingcontrol so that the focus position of the objective lens will notdeviate from the target recording layer by driving the objective lensforcibly; judging whether the focus position of the objective lens hasbeen pulled into the target recording layer by the control of preventingthe focus position of the objective lens from deviating from the targetrecording layer; if it is judged that the focus position of theobjective lens has not been pulled into the target recording layer,performing a second focus jump; and if it is judged that the focusposition of the objective lens has been pulled into the target recordinglayer: moving a laser spot to a target recording start position in thetarget recording layer, switching the laser power from the low power tothe high power, and restarting data recording.
 7. The optical discapparatus according to claim 1, wherein the focus jump is necessary in acase where during continuous data recording there occurs data whoseaddresses bridge two recording layers, and the focus jump is started inprocessing an address portion that is not to be recorded on the discafter writing of a data portion.
 8. The optical disc apparatus accordingto claim 3 wherein the drive voltage generating means generates: a firstvoltage value as an acceleration voltage that causes the objective lensto approach the disc and a second voltage value as a decelerationvoltage that causes the objective lens to go away from the disc inmoving the objective lens from a first recording layer to a secondrecording layer that is more distant from the objective lens than thefirst recording layer is, and a third voltage value as an accelerationvoltage that causes the objective lens to go away from the disc and afourth voltage value as a deceleration voltage that causes the objectivelens to approach the disc in moving the objective lens from a thirdrecording layer to a fourth recording layer that is closer to theobjective lens than the third recording layer is.
 9. The optical discapparatus according to claim 2, wherein the focus jump is necessary in acase where during continuous data recording there occurs data whoseaddresses bridge two recording layers, and the focus jump is started inprocessing an address portion that is not to be recorded on the discafter writing of a data portion.
 10. The optical disc apparatusaccording to claim 3, wherein the focus jump is necessary in a casewhere during continuous data recording there occurs data whose addressesbridge two recording layers, and the focus jump is started in processingan address portion that is not to be recorded on the disc after writingof a data portion.
 11. The optical disc apparatus according to claim 4,wherein the focus jump is necessary in a case where during continuousdata recording there occurs data whose addresses bridge two recordinglayers, and the focus jump is started in processing an address portionthat is not to be recorded on the disc after writing of a data portion.